当我们切换linux终端时，背后发生了什么？_linux consw

虚拟终端（/dev/tty*）

当用户登录时，使用的是虚拟终端。使用Ctcl+Alt+[F1—F6]组合键时，我们就可以切换到tty1、tty2、tty3等上面去。此时显示器的内容发生了变化，那这个和显卡驱动，是如何关联的？

事实上，我们切换的时候需要调用到显卡驱动中的接口，重新设置显示器的输出，也就是定位到不同的显存内容，并且原理的显存应该保留，以便切换回来后依然是原来的内容。

显示器的终端驱动drivers/tty/vt/vt.c：

切换的试过键盘接收到中断，调用vt驱动中的schedule_console_callback.启动工作项。

void schedule_console_callback(void)
{
	schedule_work(&console_work);
}

static DECLARE_WORK(console_work, console_callback);

从键盘输入ctrl_alt_fN 发生的dump_stack：

（关于键盘驱动可参考Linux input系统数据上报流程【转】 - sky-heaven - 博客园 (cnblogs.com)）

[  754.930558] ===schedule_console_callback
[  754.930560] CPU: 6 PID: 1440 Comm: kworker/6:1 Not tainted 4.19.152+ #19
[  754.930561] Hardware name: PHYTIUM LTD D2000/D2000, BIOS
[  754.930563] Workqueue: events console_callback
[  754.930564] Call trace:
[  754.930565]  dump_backtrace+0x0/0x198
[  754.930566]  show_stack+0x24/0x30
[  754.930568]  dump_stack+0x9c/0xd0
[  754.930569]  schedule_console_callback+0x20/0x44
[  754.930570]  kbd_event+0xc0/0x4b8
[  754.930572]  input_to_handler+0xf0/0x118
[  754.930573]  input_pass_values.part.2+0x130/0x148
[  754.930575]  input_handle_event+0x114/0x548
[  754.930576]  input_event+0x7c/0xb0
[  754.930578]  hidinput_report_event+0x4c/0x68
[  754.930579]  hid_report_raw_event+0x210/0x400
[  754.930581]  hid_input_report+0x104/0x1b0
[  754.930582]  hid_irq_in+0x94/0x230
[  754.930584]  __usb_hcd_giveback_urb+0x7c/0x138
[  754.930585]  usb_hcd_giveback_urb+0x168/0x188
[  754.930587]  xhci_giveback_urb_in_irq.isra.22+0x98/0x160
[  754.930589]  xhci_td_cleanup+0x118/0x1a8
[  754.930591]  finish_td+0xf8/0x130
[  754.930592]  handle_tx_event+0x24c/0x11b8
[  754.930594]  xhci_irq+0x1fc/0x4a0
[  754.930596]  xhci_msi_irq+0x20/0x118
[  754.930597]  __handle_irq_event_percpu+0x78/0x248
[  754.930599]  handle_irq_event_percpu+0x40/0x98
[  754.930601]  handle_irq_event+0x50/0xd0
[  754.930602]  handle_fasteoi_irq+0xd4/0x1a8
[  754.930604]  generic_handle_irq+0x34/0x50
[  754.930606]  __handle_domain_irq+0x6c/0xc0
[  754.930607]  gic_handle_irq+0x84/0x180
[  754.930608]  el1_irq+0xb0/0x140
[  754.930610]  console_unlock+0x374/0x590
[  754.930611]  console_callback+0xa4/0x158
[  754.930613]  process_one_work+0x1ac/0x3e0
[  754.930614]  worker_thread+0x44/0x448
[  754.930616]  kthread+0x130/0x138
[  754.930617]  ret_from_fork+0x10/0x18

后续内容和drm驱动相关的dump_stack：

1:创建agetty程序时通过console_callback，系统起来的时候没有agetty。切换的时候tty驱动会创建该进程：

[  220.466929] ===ast_primary_plane_helper_atomic_check 
[  220.466935] CPU: 5 PID: 1941 Comm: kworker/5:3 Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  220.466937] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  220.466950] Workqueue: events console_callback
[  220.466953] Call trace:
[  220.466958]  dump_backtrace+0x0/0x210
[  220.466960]  show_stack+0x20/0x68
[  220.466965]  dump_stack+0xd8/0x130
[  220.466971]  ast_primary_plane_helper_atomic_check+0x3c/0xa8 [ast]
[  220.467003]  drm_atomic_helper_check_planes+0xe0/0x200 [drm_kms_helper]
[  220.467020]  drm_atomic_helper_check+0x58/0xb0 [drm_kms_helper]
[  220.467077]  drm_atomic_check_only+0x454/0x770 [drm]
[  220.467114]  drm_atomic_commit+0x24/0x68 [drm]
[  220.467146]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  220.467175]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  220.467201]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  220.467205]  fb_pan_display+0x94/0x108
[  220.467208]  bit_update_start+0x28/0x50
[  220.467210]  fbcon_switch+0x320/0x4b0
[  220.467212]  redraw_screen+0x148/0x280
[  220.467215]  complete_change_console+0x50/0xf0
[  220.467217]  change_console+0x74/0xa8
[  220.467219]  console_callback+0x158/0x170
[  220.467223]  process_one_work+0x1c0/0x428
[  220.467224]  worker_thread+0x158/0x510
[  220.467227]  kthread+0x118/0x120
[  220.467230]  ret_from_fork+0x10/0x18
[  220.467234] ===ast_crtc_helper_atomic_check 
[  220.467237] CPU: 5 PID: 1941 Comm: kworker/5:3 Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  220.467239] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  220.467246] Workqueue: events console_callback
[  220.467248] Call trace:
[  220.467250]  dump_backtrace+0x0/0x210
[  220.467252]  show_stack+0x20/0x68
[  220.467254]  dump_stack+0xd8/0x130
[  220.467258]  ast_crtc_helper_atomic_check+0x30/0x1454 [ast]
[  220.467275]  drm_atomic_helper_check_planes+0x140/0x200 [drm_kms_helper]
[  220.467289]  drm_atomic_helper_check+0x58/0xb0 [drm_kms_helper]
[  220.467326]  drm_atomic_check_only+0x454/0x770 [drm]
[  220.467356]  drm_atomic_commit+0x24/0x68 [drm]
[  220.467384]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  220.467414]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  220.467432]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  220.467434]  fb_pan_display+0x94/0x108
[  220.467437]  bit_update_start+0x28/0x50
[  220.467439]  fbcon_switch+0x320/0x4b0
[  220.467441]  redraw_screen+0x148/0x280
[  220.467444]  complete_change_console+0x50/0xf0
[  220.467446]  change_console+0x74/0xa8
[  220.467448]  console_callback+0x158/0x170
[  220.467450]  process_one_work+0x1c0/0x428
[  220.467451]  worker_thread+0x158/0x510
[  220.467454]  kthread+0x118/0x120
[  220.467456]  ret_from_fork+0x10/0x18
[  220.467459] ===drm_atomic_helper_commit
[  220.467462] CPU: 5 PID: 1941 Comm: kworker/5:3 Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  220.467463] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  220.467470] Workqueue: events console_callback
[  220.467472] Call trace:
[  220.467474]  dump_backtrace+0x0/0x210
[  220.467475]  show_stack+0x20/0x68
[  220.467477]  dump_stack+0xd8/0x130
[  220.467492]  drm_atomic_helper_commit+0x3c/0x14c [drm_kms_helper]
[  220.467524]  drm_atomic_commit+0x54/0x68 [drm]
[  220.467554]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  220.467584]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  220.467600]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  220.467603]  fb_pan_display+0x94/0x108
[  220.467605]  bit_update_start+0x28/0x50
[  220.467607]  fbcon_switch+0x320/0x4b0
[  220.467610]  redraw_screen+0x148/0x280
[  220.467612]  complete_change_console+0x50/0xf0
[  220.467614]  change_console+0x74/0xa8
[  220.467616]  console_callback+0x158/0x170
[  220.467618]  process_one_work+0x1c0/0x428
[  220.467620]  worker_thread+0x158/0x510
[  220.467622]  kthread+0x118/0x120
[  220.467624]  ret_from_fork+0x10/0x18
[  220.467635] ===drm_atomic_helper_commit_tail_rpm
[  220.467638] CPU: 5 PID: 1941 Comm: kworker/5:3 Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  220.467639] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  220.467646] Workqueue: events console_callback
[  220.467648] Call trace:
[  220.467650]  dump_backtrace+0x0/0x210
[  220.467651]  show_stack+0x20/0x68
[  220.467654]  dump_stack+0xd8/0x130
[  220.467669]  drm_atomic_helper_commit_tail_rpm+0x30/0x98 [drm_kms_helper]
[  220.467683]  commit_tail+0xac/0x1a8 [drm_kms_helper]
[  220.467697]  drm_atomic_helper_commit+0x120/0x14c [drm_kms_helper]
[  220.467731]  drm_atomic_commit+0x54/0x68 [drm]
[  220.467762]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  220.467791]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  220.467809]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  220.467811]  fb_pan_display+0x94/0x108
[  220.467814]  bit_update_start+0x28/0x50
[  220.467816]  fbcon_switch+0x320/0x4b0
[  220.467818]  redraw_screen+0x148/0x280
[  220.467820]  complete_change_console+0x50/0xf0
[  220.467822]  change_console+0x74/0xa8
[  220.467824]  console_callback+0x158/0x170
[  220.467827]  process_one_work+0x1c0/0x428
[  220.467828]  worker_thread+0x158/0x510
[  220.467830]  kthread+0x118/0x120
[  220.467832]  ret_from_fork+0x10/0x18
[  220.467836] ===ast_primary_plane_helper_atomic_update gpu_addr 0 comm kworker/5:3 
[  220.467838] CPU: 5 PID: 1941 Comm: kworker/5:3 Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  220.467840] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  220.467847] Workqueue: events console_callback
[  220.467849] Call trace:
[  220.467851]  dump_backtrace+0x0/0x210
[  220.467852]  show_stack+0x20/0x68
[  220.467855]  dump_stack+0xd8/0x130
[  220.467859]  ast_primary_plane_helper_atomic_update+0x188/0x2b0 [ast]
[  220.467875]  drm_atomic_helper_commit_planes+0xf0/0x218 [drm_kms_helper]
[  220.467888]  drm_atomic_helper_commit_tail_rpm+0x58/0x98 [drm_kms_helper]
[  220.467902]  commit_tail+0xac/0x1a8 [drm_kms_helper]
[  220.467915]  drm_atomic_helper_commit+0x120/0x14c [drm_kms_helper]
[  220.467949]  drm_atomic_commit+0x54/0x68 [drm]
[  220.467979]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  220.468007]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  220.468022]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  220.468024]  fb_pan_display+0x94/0x108
[  220.468027]  bit_update_start+0x28/0x50
[  220.468029]  fbcon_switch+0x320/0x4b0
[  220.468031]  redraw_screen+0x148/0x280
[  220.468033]  complete_change_console+0x50/0xf0
[  220.468035]  change_console+0x74/0xa8
[  220.468037]  console_callback+0x158/0x170
[  220.468040]  process_one_work+0x1c0/0x428
[  220.468041]  worker_thread+0x158/0x510
[  220.468043]  kthread+0x118/0x120
[  220.468045]  ret_from_fork+0x10/0x18
[  220.468059] ===ast_crtc_helper_atomic_flush
[  220.468062] CPU: 5 PID: 1941 Comm: kworker/5:3 Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  220.468063] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  220.468070] Workqueue: events console_callback
[  220.468072] Call trace:
[  220.468073]  dump_backtrace+0x0/0x210
[  220.468075]  show_stack+0x20/0x68
[  220.468077]  dump_stack+0xd8/0x130
[  220.468081]  ast_crtc_helper_atomic_flush+0x3c/0x160 [ast]
[  220.468097]  drm_atomic_helper_commit_planes+0x15c/0x218 [drm_kms_helper]
[  220.468112]  drm_atomic_helper_commit_tail_rpm+0x58/0x98 [drm_kms_helper]
[  220.468125]  commit_tail+0xac/0x1a8 [drm_kms_helper]
[  220.468139]  drm_atomic_helper_commit+0x120/0x14c [drm_kms_helper]
[  220.468175]  drm_atomic_commit+0x54/0x68 [drm]
[  220.468206]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  220.468237]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  220.468256]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  220.468258]  fb_pan_display+0x94/0x108
[  220.468261]  bit_update_start+0x28/0x50
[  220.468263]  fbcon_switch+0x320/0x4b0
[  220.468265]  redraw_screen+0x148/0x280
[  220.468268]  complete_change_console+0x50/0xf0
[  220.468270]  change_console+0x74/0xa8
[  220.468272]  console_callback+0x158/0x170
[  220.468275]  process_one_work+0x1c0/0x428
[  220.468276]  worker_thread+0x158/0x510
[  220.468278]  kthread+0x118/0x120
[  220.468280]  ret_from_fork+0x10/0x18

总结流程： 

console_callback
	change_console
		complete_change_console
			redraw_screen
				fbcon_switch
					bit_update_start
						fb_pan_display
							drm_fb_helper_pan_display
								drm_client_modeset_commit_locked
									drm_client_modeset_commit_atomic
										drm_atomic_commit
											drm_atomic_check_only
												drm_atomic_helper_check
													drm_atomic_helper_check_planes   //调用drm_xx_helper_funcs->atomic_check
														ast_primary_plane_helper_atomic_check  //plane
														ast_crtc_helper_atomic_check		    //crtc
											
											drm_atomic_helper_commit
												commit_tail
													drm_atomic_helper_commit_tail_rpm
														drm_atomic_helper_commit_planes //调用crtc的atomic_begin。plane的atomic_update，crtc的atomic_flush
															ast_primary_plane_helper_atomic_update
															ast_crtc_helper_atomic_flush

上面是ctrl+alt+Fx 后 直到agetty程序启动前。下面是agetty启动后字符界面出现内容，如果之前以及切换过，也就是agetty起来了，则不会走下面的了。

2:agetty程序启动后tty_write

[  221.249372] ===ast_primary_plane_helper_atomic_check 
[  221.249386] CPU: 105 PID: 3477 Comm: (agetty) Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  221.249389] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  221.249393] Call trace:
[  221.249409]  dump_backtrace+0x0/0x210
[  221.249412]  show_stack+0x20/0x68
[  221.249423]  dump_stack+0xd8/0x130
[  221.249436]  ast_primary_plane_helper_atomic_check+0x3c/0xa8 [ast]
[  221.249503]  drm_atomic_helper_check_planes+0xe0/0x200 [drm_kms_helper]
[  221.249522]  drm_atomic_helper_check+0x58/0xb0 [drm_kms_helper]
[  221.249635]  drm_atomic_check_only+0x454/0x770 [drm]
[  221.249678]  drm_atomic_commit+0x24/0x68 [drm]
[  221.249708]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  221.249736]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  221.249774]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  221.249785]  fb_pan_display+0x94/0x108
[  221.249802]  bit_update_start+0x28/0x50
[  221.249805]  fbcon_switch+0x320/0x4b0
[  221.249814]  csi_J+0x1c8/0x1d8
[  221.249819]  do_con_trol+0x1388/0x1478
[  221.249821]  do_con_write+0x208/0x930
[  221.249823]  con_write+0x20/0x70
[  221.249831]  n_tty_write+0x19c/0x468
[  221.249835]  tty_write+0x184/0x340
[  221.249842]  vfs_write+0xc8/0x2b8
[  221.249844]  ksys_write+0x70/0xf8
[  221.249846]  __arm64_sys_write+0x24/0x30
[  221.249854]  el0_svc_common.constprop.3+0xa4/0x1f8
[  221.249857]  do_el0_svc+0x2c/0x98
[  221.249862]  el0_svc+0x28/0x68
[  221.249864]  el0_sync_handler+0x90/0xb8
[  221.249868]  el0_sync+0x158/0x180
[  221.249875] ===ast_crtc_helper_atomic_check 
[  221.249878] CPU: 105 PID: 3477 Comm: (agetty) Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  221.249881] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  221.249883] Call trace:
[  221.249885]  dump_backtrace+0x0/0x210
[  221.249887]  show_stack+0x20/0x68
[  221.249889]  dump_stack+0xd8/0x130
[  221.249895]  ast_crtc_helper_atomic_check+0x30/0x1454 [ast]
[  221.249915]  drm_atomic_helper_check_planes+0x140/0x200 [drm_kms_helper]
[  221.249929]  drm_atomic_helper_check+0x58/0xb0 [drm_kms_helper]
[  221.249975]  drm_atomic_check_only+0x454/0x770 [drm]
[  221.250016]  drm_atomic_commit+0x24/0x68 [drm]
[  221.250047]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  221.250089]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  221.250125]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  221.250129]  fb_pan_display+0x94/0x108
[  221.250134]  bit_update_start+0x28/0x50
[  221.250136]  fbcon_switch+0x320/0x4b0
[  221.250140]  csi_J+0x1c8/0x1d8
[  221.250143]  do_con_trol+0x1388/0x1478
[  221.250145]  do_con_write+0x208/0x930
[  221.250147]  con_write+0x20/0x70
[  221.250151]  n_tty_write+0x19c/0x468
[  221.250154]  tty_write+0x184/0x340
[  221.250157]  vfs_write+0xc8/0x2b8
[  221.250159]  ksys_write+0x70/0xf8
[  221.250161]  __arm64_sys_write+0x24/0x30
[  221.250166]  el0_svc_common.constprop.3+0xa4/0x1f8
[  221.250169]  do_el0_svc+0x2c/0x98
[  221.250176]  el0_svc+0x28/0x68
[  221.250178]  el0_sync_handler+0x90/0xb8
[  221.250181]  el0_sync+0x158/0x180
[  221.250278] ===drm_atomic_helper_commit
[  221.250288] CPU: 105 PID: 3477 Comm: (agetty) Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  221.250290] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  221.250292] Call trace:
[  221.250298]  dump_backtrace+0x0/0x210
[  221.250300]  show_stack+0x20/0x68
[  221.250304]  dump_stack+0xd8/0x130
[  221.250345]  drm_atomic_helper_commit+0x3c/0x14c [drm_kms_helper]
[  221.250438]  drm_atomic_commit+0x54/0x68 [drm]
[  221.250487]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  221.250535]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  221.250567]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  221.250572]  fb_pan_display+0x94/0x108
[  221.250576]  bit_update_start+0x28/0x50
[  221.250578]  fbcon_switch+0x320/0x4b0
[  221.250583]  csi_J+0x1c8/0x1d8
[  221.250585]  do_con_trol+0x1388/0x1478
[  221.250587]  do_con_write+0x208/0x930
[  221.250590]  con_write+0x20/0x70
[  221.250596]  n_tty_write+0x19c/0x468
[  221.250600]  tty_write+0x184/0x340
[  221.250606]  vfs_write+0xc8/0x2b8
[  221.250608]  ksys_write+0x70/0xf8
[  221.250610]  __arm64_sys_write+0x24/0x30
[  221.250619]  el0_svc_common.constprop.3+0xa4/0x1f8
[  221.250621]  do_el0_svc+0x2c/0x98
[  221.250624]  el0_svc+0x28/0x68
[  221.250626]  el0_sync_handler+0x90/0xb8
[  221.250629]  el0_sync+0x158/0x180
[  221.250653] ===drm_atomic_helper_commit_tail_rpm
[  221.250656] CPU: 105 PID: 3477 Comm: (agetty) Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  221.250658] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  221.250659] Call trace:
[  221.250662]  dump_backtrace+0x0/0x210
[  221.250664]  show_stack+0x20/0x68
[  221.250668]  dump_stack+0xd8/0x130
[  221.250692]  drm_atomic_helper_commit_tail_rpm+0x30/0x98 [drm_kms_helper]
[  221.250708]  commit_tail+0xac/0x1a8 [drm_kms_helper]
[  221.250726]  drm_atomic_helper_commit+0x120/0x14c [drm_kms_helper]
[  221.250798]  drm_atomic_commit+0x54/0x68 [drm]
[  221.250840]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  221.250881]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  221.250917]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  221.250919]  fb_pan_display+0x94/0x108
[  221.250922]  bit_update_start+0x28/0x50
[  221.250924]  fbcon_switch+0x320/0x4b0
[  221.250927]  csi_J+0x1c8/0x1d8
[  221.250930]  do_con_trol+0x1388/0x1478
[  221.250932]  do_con_write+0x208/0x930
[  221.250935]  con_write+0x20/0x70
[  221.250940]  n_tty_write+0x19c/0x468
[  221.250942]  tty_write+0x184/0x340
[  221.250946]  vfs_write+0xc8/0x2b8
[  221.250947]  ksys_write+0x70/0xf8
[  221.250949]  __arm64_sys_write+0x24/0x30
[  221.250954]  el0_svc_common.constprop.3+0xa4/0x1f8
[  221.250956]  do_el0_svc+0x2c/0x98
[  221.250961]  el0_svc+0x28/0x68
[  221.250967]  el0_sync_handler+0x90/0xb8
[  221.250969]  el0_sync+0x158/0x180
[  221.250979] ===ast_primary_plane_helper_atomic_update gpu_addr 0 comm (agetty) 
[  221.250982] CPU: 105 PID: 3477 Comm: (agetty) Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  221.250984] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  221.250986] Call trace:
[  221.250987]  dump_backtrace+0x0/0x210
[  221.250989]  show_stack+0x20/0x68
[  221.250993]  dump_stack+0xd8/0x130
[  221.251002]  ast_primary_plane_helper_atomic_update+0x188/0x2b0 [ast]
[  221.251024]  drm_atomic_helper_commit_planes+0xf0/0x218 [drm_kms_helper]
[  221.251038]  drm_atomic_helper_commit_tail_rpm+0x58/0x98 [drm_kms_helper]
[  221.251051]  commit_tail+0xac/0x1a8 [drm_kms_helper]
[  221.251065]  drm_atomic_helper_commit+0x120/0x14c [drm_kms_helper]
[  221.251122]  drm_atomic_commit+0x54/0x68 [drm]
[  221.251159]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  221.251187]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  221.251205]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  221.251207]  fb_pan_display+0x94/0x108
[  221.251211]  bit_update_start+0x28/0x50
[  221.251217]  fbcon_switch+0x320/0x4b0
[  221.251220]  csi_J+0x1c8/0x1d8
[  221.251222]  do_con_trol+0x1388/0x1478
[  221.251223]  do_con_write+0x208/0x930
[  221.251232]  con_write+0x20/0x70
[  221.251235]  n_tty_write+0x19c/0x468
[  221.251237]  tty_write+0x184/0x340
[  221.251240]  vfs_write+0xc8/0x2b8
[  221.251242]  ksys_write+0x70/0xf8
[  221.251244]  __arm64_sys_write+0x24/0x30
[  221.251248]  el0_svc_common.constprop.3+0xa4/0x1f8
[  221.251250]  do_el0_svc+0x2c/0x98
[  221.251252]  el0_svc+0x28/0x68
[  221.251254]  el0_sync_handler+0x90/0xb8
[  221.251257]  el0_sync+0x158/0x180
[  221.251509] ===ast_crtc_helper_atomic_flush
[  221.251513] CPU: 105 PID: 3477 Comm: (agetty) Kdump: loaded Tainted: G            E     5.10.0.anolis+ #8
[  221.251514] Hardware name: PHYTIUM LTD Phytium S2500/64/Phytium S2500/64, BIOS V2.2 Feb  9 2021
[  221.251515] Call trace:
[  221.251517]  dump_backtrace+0x0/0x210
[  221.251519]  show_stack+0x20/0x68
[  221.251521]  dump_stack+0xd8/0x130
[  221.251526]  ast_crtc_helper_atomic_flush+0x3c/0x160 [ast]
[  221.251552]  drm_atomic_helper_commit_planes+0x15c/0x218 [drm_kms_helper]
[  221.251569]  drm_atomic_helper_commit_tail_rpm+0x58/0x98 [drm_kms_helper]
[  221.251590]  commit_tail+0xac/0x1a8 [drm_kms_helper]
[  221.251612]  drm_atomic_helper_commit+0x120/0x14c [drm_kms_helper]
[  221.251677]  drm_atomic_commit+0x54/0x68 [drm]
[  221.251718]  drm_client_modeset_commit_atomic.isra.14+0x1ec/0x290 [drm]
[  221.251755]  drm_client_modeset_commit_locked+0x68/0x1a8 [drm]
[  221.251782]  drm_fb_helper_pan_display+0xc0/0x1e0 [drm_kms_helper]
[  221.251786]  fb_pan_display+0x94/0x108
[  221.251789]  bit_update_start+0x28/0x50
[  221.251792]  fbcon_switch+0x320/0x4b0
[  221.251795]  csi_J+0x1c8/0x1d8
[  221.251798]  do_con_trol+0x1388/0x1478
[  221.251800]  do_con_write+0x208/0x930
[  221.251807]  con_write+0x20/0x70
[  221.251810]  n_tty_write+0x19c/0x468
[  221.251818]  tty_write+0x184/0x340
[  221.251821]  vfs_write+0xc8/0x2b8
[  221.251823]  ksys_write+0x70/0xf8
[  221.251825]  __arm64_sys_write+0x24/0x30
[  221.251829]  el0_svc_common.constprop.3+0xa4/0x1f8
[  221.251831]  do_el0_svc+0x2c/0x98
[  221.251835]  el0_svc+0x28/0x68
[  221.251838]  el0_sync_handler+0x90/0xb8
[  221.251840]  el0_sync+0x158/0x180

tty参考：Linux 终端初始化 console_init 及 tty 驱动框架 - 腾讯云开发者社区-腾讯云 (tencent.com)

printk打印：【转载】从printk到framebuff - hydah - 博客园 (cnblogs.com)

“printk打印需要调用console_driver。而vt.c代表的显示器控制台也注册了，因此prink会往显示器输出，最终调用consw的接口来写。consw的提供有很多在drivers/video下面

显示器输出需要结合framebuffer。因此framebuffr需要调用vt提供的接口注册(do_take_over_console)，将consw fb_con注册。vt的打印调用consw的接口来操作framebuffer，再操作drm。。。。”

static int __init con_init(void)
{
...
#ifdef CONFIG_VT_CONSOLE
	register_console(&vt_console_driver);
#endif
	return 0;
}
 
 
static struct console vt_console_driver = {
	.name		= "tty",
	.write		= vt_console_print, //printk调用这个写函数
	.device		= vt_console_device,
	.unblank	= unblank_screen,
	.flags		= CON_PRINTBUFFER,
	.index		= -1,
};
 
 
static void vt_console_print(struct console *co, const char *b, unsigned count)
{
	struct vc_data *vc = vc_cons[fg_console].d;
 
    vc->vc_sw->con_putcs(vc, start, cnt, vc->vc_y, vc->vc_x); //调用consw 的con_putc函数
 
}

drivers/video 下提供console的有多个：

 其中我们如果是framebuffer的话，在fbdev下面。通过register_framebuffer通用注册。efifb和drm都是一样。

[   12.653736]  do_take_over_console+0x158/0x240   //drivers/tty/vt/vt.c
[   12.653739]  do_fbcon_takeover+0x74/0xe0         //drivers/tty/vt/vt.c
[   12.653741]  fbcon_event_notify+0x858/0x8c8
[   12.653744]  notifier_call_chain+0x5c/0x98
[   12.653746]  blocking_notifier_call_chain+0x64/0xa0
[   12.653748]  fb_notifier_call_chain+0x30/0x40
[   12.653750]  register_framebuffer+0x2b4/0x340   //drivers/video/fbdev/core/fbmem.c

但如果是framebuffer的话 (在fbdev下面)，只能支持一个(efifb或者显卡驱动注册的register_framebuffer)。因为vt.c中的registered_con_driver总共16个且不能重复。意味着如果efifb注册了。后面的将无法注册。除非将efifb干掉。因此。通过drm启动后efifb会被移除。参考后面的do_take_over_console

可以博客写了tty的显示过程。

(126条消息) tty0 显示的过程_tiantao2012的博客-CSDN博客

显示控制台的驱动是：drivers/tty/vt/vt.c：

其中定义了一个工作队列：console_work

​
static DECLARE_WORK(console_work, console_callback); //work工作项
 
console_callback是其工作函数。每次切换终端将调用： 对应了前面的dump_stack
 
void schedule_console_callback(void)
{
	schedule_work(&console_work);  //调用
}
 
 
static void console_callback(struct work_struct *ignored)
{
	console_lock();
 
	if (want_console >= 0) {
		if (want_console != fg_console &&
		    vc_cons_allocated(want_console)) {
			hide_cursor(vc_cons[fg_console].d);
			change_console(vc_cons[want_console].d); //执行切换。
			/* we only changed when the console had already
			   been allocated - a new console is not created
			   in an interrupt routine */
		}
		want_console = -1;
	}
	if (do_poke_blanked_console) { /* do not unblank for a LED change */
		do_poke_blanked_console = 0;
		poke_blanked_console();
	}
	if (scrollback_delta) {
		struct vc_data *vc = vc_cons[fg_console].d;
		clear_selection();
		if (vc->vc_mode == KD_TEXT && vc->vc_sw->con_scrolldelta)
			vc->vc_sw->con_scrolldelta(vc, scrollback_delta);
		scrollback_delta = 0;
	}
	if (blank_timer_expired) {
		do_blank_screen(0);
		blank_timer_expired = 0;
	}
	notify_update(vc_cons[fg_console].d);
 
	console_unlock();
}
 
​

vc_cons：

struct vc vc_cons [MAX_NR_CONSOLES]; 总共63个从tty1->tty63 

#define MIN_NR_CONSOLES 1       /* must be at least 1 */
#define MAX_NR_CONSOLES	63	/* serial lines start at 64 */

 其中tty和tty0 是特殊用。Ctrl+alt+fx 对应ttyx

下面是/dev/目录下的ttyX。 drivers/vt/vt.c对应的是ttyX

再看一下drivers/vt/vt.c 中的

#define MAX_NR_CON_DRIVER 16
 
struct con_driver {
	const struct consw *con;
	const char *desc;
	struct device *dev;
	int node;
	int first;
	int last;
	int flag;
};
 
static struct con_driver registered_con_driver[MAX_NR_CON_DRIVER]; //总共可以注册的consw为16个。且需要不同。比如fb_con。sti_con，mda_con，newport_con，dummy_con等等
const struct consw *conswitchp;
 
 
 
struct vc vc_cons [MAX_NR_CONSOLES]; //总共的终端时63个。
 
#ifndef VT_SINGLE_DRIVER
static const struct consw *con_driver_map[MAX_NR_CONSOLES];  //总共63个，在do_bind_con_driver函数中会预先设置consw。参考 do_bind_con_driver
#endif

do_take_over_console 注册：

int do_take_over_console(const struct consw *csw, int first, int last, int deflt)
{
	int err;
 
	err = do_register_con_driver(csw, first, last); // 注册consw。如果已经有相同的consw将返回失败。
	/*
	 * If we get an busy error we still want to bind the console driver
	 * and return success, as we may have unbound the console driver
	 * but not unregistered it.
	 */
	if (err == -EBUSY)
		err = 0;
	if (!err)
		do_bind_con_driver(csw, first, last, deflt);
 
	return err;
}

do_register_con_driver：注册consw到 registered_con_driver中。无法重复注册（register_framebuffer背后使用的consw是一个fb_con）。

static int do_register_con_driver(const struct consw *csw, int first, int last)
{
	struct module *owner = csw->owner;
	struct con_driver *con_driver;
	const char *desc;
	int i, retval;
 
	WARN_CONSOLE_UNLOCKED();
 
	if (!try_module_get(owner))
		return -ENODEV;
 
	for (i = 0; i < MAX_NR_CON_DRIVER; i++) {
		con_driver = &registered_con_driver[i]; 
 
		/* already registered */
		if (con_driver->con == csw) {  //如果已经存在。。。。这返回。
			retval = -EBUSY;
			goto err;
		}
	}
 
	desc = csw->con_startup();
	if (!desc) {
		retval = -ENODEV;
		goto err;
	}
 
	retval = -EINVAL;
 
	for (i = 0; i < MAX_NR_CON_DRIVER; i++) {
		con_driver = &registered_con_driver[i];
 
		if (con_driver->con == NULL &&
		    !(con_driver->flag & CON_DRIVER_FLAG_ZOMBIE)) {
			con_driver->con = csw;
			con_driver->desc = desc;
			con_driver->node = i;
			con_driver->flag = CON_DRIVER_FLAG_MODULE |
			                   CON_DRIVER_FLAG_INIT;
			con_driver->first = first;
			con_driver->last = last;
			retval = 0;
			break;
		}
	}
 
	if (retval)
		goto err;
 
	con_driver->dev =
		device_create_with_groups(vtconsole_class, NULL,
					  MKDEV(0, con_driver->node),
					  con_driver, con_dev_groups,
					  "vtcon%i", con_driver->node);
	if (IS_ERR(con_driver->dev)) {
		pr_warn("Unable to create device for %s; errno = %ld\n",
			con_driver->desc, PTR_ERR(con_driver->dev));
		con_driver->dev = NULL;
	} else {
		vtconsole_init_device(con_driver);
	}
 
err:
	module_put(owner);
	return retval;
}

do_bind_con_driver：绑定从first到last范围内的所有ttyX设备的consw。当然前提是该设备已经创建。 

static int do_bind_con_driver(const struct consw *csw, int first, int last,
			   int deflt)   //deflt是否为默认consw。通常注册的都是，比如fb_con
{
	struct module *owner = csw->owner;
	const char *desc = NULL;
	struct con_driver *con_driver;
	int i, j = -1, k = -1, retval = -ENODEV;
 
	if (!try_module_get(owner))
		return -ENODEV;
 
	WARN_CONSOLE_UNLOCKED();
 
	/* check if driver is registered */
	for (i = 0; i < MAX_NR_CON_DRIVER; i++) {
		con_driver = &registered_con_driver[i];
 
		if (con_driver->con == csw) {
			desc = con_driver->desc;
			retval = 0;
			break;
		}
	}
 
	if (retval)
		goto err;
 
	if (!(con_driver->flag & CON_DRIVER_FLAG_INIT)) {
		csw->con_startup();
		con_driver->flag |= CON_DRIVER_FLAG_INIT;
	}
 
	if (deflt) {
		if (conswitchp)
			module_put(conswitchp->owner);
 
		__module_get(owner);
		conswitchp = csw;  //替换为默认
	}
 
	first = max(first, con_driver->first);
	last = min(last, con_driver->last);
 
	for (i = first; i <= last; i++) {
		int old_was_color;
		struct vc_data *vc = vc_cons[i].d;   //为63个ttyX 初始化数据
 
		if (con_driver_map[i])
			module_put(con_driver_map[i]->owner);
		__module_get(owner);
		con_driver_map[i] = csw;
 
		if (!vc || !vc->vc_sw)  //如果对应的ttyX没有创建，则不需要，一般系统并不会创建全部。
			continue;
 
		j = i;
 
		if (con_is_visible(vc)) {
			k = i;
			save_screen(vc);
		}
 
		old_was_color = vc->vc_can_do_color;
		vc->vc_sw->con_deinit(vc);
		vc->vc_origin = (unsigned long)vc->vc_screenbuf;
		visual_init(vc, i, 0); //配置
		set_origin(vc);
		update_attr(vc);
 
		/* If the console changed between mono <-> color, then
		 * the attributes in the screenbuf will be wrong.  The
		 * following resets all attributes to something sane.
		 */
		if (old_was_color != vc->vc_can_do_color)
			clear_buffer_attributes(vc);
	}
 
	pr_info("Console: switching ");
	if (!deflt)
		pr_cont("consoles %d-%d ", first + 1, last + 1);
	if (j >= 0) {
		struct vc_data *vc = vc_cons[j].d;
 
		pr_cont("to %s %s %dx%d\n",
			vc->vc_can_do_color ? "colour" : "mono",
			desc, vc->vc_cols, vc->vc_rows);
 
		if (k >= 0) {
			vc = vc_cons[k].d;
			update_screen(vc);
		}
	} else {
		pr_cont("to %s\n", desc);
	}
 
	retval = 0;
err:
	module_put(owner);
	return retval;
};

do_bind_con_driver后谁最后调用，将使用谁的consw，如果通过register_framebuffer注册的consw是用一个，因此如果efifb先注册，则使用efifb，drm注册的可能用不到。

 

打印：do_bind_con_driver

[   17.211099] ===fbcon_fb_registered idx 0 info_idx -1 
[   17.211210] ===do_bind_con_driver ffff800010e15818 first 0 last 62 
[   17.211212] ===visual_init vc->vc_num 0 vc->vc_sw ffff800010e15818 
[   17.211221] ===fbcon_init 
[   21.177371] ===do_bind_con_driver ffff800010e14368 first 0 last 62 
[   21.177376] ===visual_init vc->vc_num 0 vc->vc_sw ffff800010e14368 
[   22.140147] ===fbcon_fb_registered idx 0 info_idx -1 
[   22.141070] ===do_bind_con_driver ffff800010e15818 first 0 last 62 
[   22.141073] ===visual_init vc->vc_num 0 vc->vc_sw ffff800010e15818 
[   22.141074] ===fbcon_init 
[  108.158192] ===visual_init vc->vc_num 1 vc->vc_sw ffff800010e15818 
[  108.158194] ===fbcon_init 
[  108.265646] ===visual_init vc->vc_num 2 vc->vc_sw ffff800010e15818 
[  108.265648] ===fbcon_init 
[  108.374542] ===visual_init vc->vc_num 3 vc->vc_sw ffff800010e15818 
[  108.374543] ===fbcon_init 
[  108.477747] ===visual_init vc->vc_num 4 vc->vc_sw ffff800010e15818 
[  108.477749] ===fbcon_init 
[  108.585954] ===visual_init vc->vc_num 5 vc->vc_sw ffff800010e15818 
[  108.585955] ===fbcon_init 

上述两个consw：分别可以打印出来，其vc_num：中0-5的6个分别代表tty1-6.

root@wzm-phytium-d2000:~# cat /proc/kallsyms |grep ffff800010e14368
ffff800010e14368 D dummy_con
root@wzm-phytium-d2000:~# cat /proc/kallsyms |grep ffff800010e15818
ffff800010e15818 d fb_con

前面分析切换终端调用fbcon_switch:

static int fbcon_switch(struct vc_data *vc)
{
        struct fb_info *info, *old_info = NULL;
        struct fbcon_ops *ops;
        struct fbcon_display *p = &fb_display[vc->vc_num];
        struct fb_var_screeninfo var;
        int i, ret, prev_console, charcnt = 256;
 
        info = registered_fb[con2fb_map[vc->vc_num]]; //根据vc_num 计算出registered_fb。进而找出fbcon_ops
 
。。。。
}

con2fb_map数组中的值在do_fbcon_takeover中设置。

static int do_fbcon_takeover(int show_logo)
{
        int err, i;
 
        if (!num_registered_fb)
                return -ENODEV;
 
        if (!show_logo)
                logo_shown = FBCON_LOGO_DONTSHOW;
 
        for (i = first_fb_vc; i <= last_fb_vc; i++)   //0-62
                con2fb_map[i] = info_idx;  //info_idx 的值通常就是0，在函数fbcon_fb_registered中先从-1 设置为0.
        err = do_take_over_console(&fb_con, first_fb_vc, last_fb_vc,
                                fbcon_is_default);
。。。
}