Re: [PATCH 1/1 2.6.15-rc4-git1] Fix switching to KD_TEXT

[Knut Petersen]

Hi Tony!

> Sorry, NAK for now.  Unless other people agree that it is okay for them
> to have an unconditional call to set_par() for every console switch. Note
> that the set_par() in some drivers is terribly slow (several seconds at least).
>
> Let's wait a few days, if nobody disagrees with you, so be it.
>
>  
Which hardware needs several seconds? I do run relatively slow hardware, 
but a complete
set_par() needs less than 0.003 seconds with cyblafb.

If there really is hardware that needs _several_ seconds for _one_ 
set_par(), then we should
really be more carefull when to call that function.

Let´s have a look at a switch from the framebuffer console to X and back.

Kernel is a 2.6.15-rc4-git1 with my current cyblafb + your patches 
needed for cyblafb
+ the patch we are talking about.

Framebuffer console is 1 with mode 1280x1024, X console is 7 and has 
been set to 1024x768
before X was activated.

Now I do run "chvt 7;sleep 3;chvt 1" on the vt 1.

   [  972.360841]  [<c0282331>] cyblafb_set_par+0x31/0xd80
   [  972.360898]  [<c027bb38>] fb_set_var+0x1e8/0x210
   [  972.360925]  [<c02742fc>] fbcon_switch+0x16c/0x690
   [  972.360951]  [<c02cc645>] redraw_screen+0xe5/0x1c0
   [  972.360989]  [<c02c811a>] complete_change_console+0x2a/0xd0
   [  972.361018]  [<c02c8203>] change_console+0x43/0x90
   [  972.361043]  [<c02ceebd>] console_callback+0xed/0x110
   [  972.361101]  [<c0127e6a>] worker_thread+0x1ba/0x260
   [  972.361143]  [<c012c115>] kthread+0x95/0xd0
   [  972.361171]  [<c0100c15>] kernel_thread_helper+0x5/0x10
   [  972.361212] cyblafb: Switching to new mode: fbset -g 1024 768 
1024 768 8 -t 15385 160 24 29 3 136 6
   [  972.363308] cyblafb: pixclock = 64.99 MHz, k/m/n 1 b 6e

   [  972.363916]  [<c0282331>] cyblafb_set_par+0x31/0xd80
   [  972.363945]  [<c0274759>] fbcon_switch+0x5c9/0x690
   [  972.363969]  [<c02cc645>] redraw_screen+0xe5/0x1c0
   [  972.363996]  [<c02c811a>] complete_change_console+0x2a/0xd0
   [  972.364023]  [<c02c8203>] change_console+0x43/0x90
   [  972.364047]  [<c02ceebd>] console_callback+0xed/0x110
   [  972.364093]  [<c0127e6a>] worker_thread+0x1ba/0x260
   [  972.364121]  [<c012c115>] kthread+0x95/0xd0
   [  972.364143]  [<c0100c15>] kernel_thread_helper+0x5/0x10
   [  972.364181] cyblafb: Switching to new mode: fbset -g 1024 768 
1024 768 8 -t 15385 160 24 29 3 136 6
   [  972.366271] cyblafb: pixclock = 64.99 MHz, k/m/n 1 b 6e
   [  972.366325] cyblafb: Switching to KD_GRAPHICS

Not so nice. Now let´s have a look at the log entries caused switching 
back to
the framebuffer console:

This call to set_par() is the result of my patch:

   [  975.751407]  [<c0282331>] cyblafb_set_par+0x31/0xd80
   [  975.751437]  [<c027bb38>] fb_set_var+0x1e8/0x210
   [  975.751462]  [<c02742fc>] fbcon_switch+0x16c/0x690
   [  975.751486]  [<c02cc645>] redraw_screen+0xe5/0x1c0
   [  975.751513]  [<c02c811a>] complete_change_console+0x2a/0xd0
   [  975.751540]  [<c02c765d>] vt_ioctl+0x103d/0x19b0
   [  975.751564]  [<c02c17eb>] tty_ioctl+0x18b/0x3d0
   [  975.751587]  [<c016e92a>] do_ioctl+0x5a/0x90
   [  975.751613]  [<c016eaeb>] vfs_ioctl+0x5b/0x1d0
   [  975.751640]  [<c016eca5>] sys_ioctl+0x45/0x70
   [  975.751666]  [<c01029a9>] syscall_call+0x7/0xb
   [  975.751702] cyblafb: Switching to new mode: fbset -g 1280 1024 
2048 4096 8 -t 7407 232 16 39 0 160 3
   [  975.753795] cyblafb: pixclock = 135.00 MHz, k/m/n 0 5 3a

Without that prior set_par() that´s the point where the enhanced cyblafb 
would
have serious problems:

   [  975.753839] cyblafb_pan_display: entry
   [  975.753853]  [<c0281f7c>] cyblafb_pan_display+0x9c/0xb0
   [  975.753883]  [<c027b911>] fb_pan_display+0x61/0xa0
   [  975.753925]  [<c027ba4f>] fb_set_var+0xff/0x210
   [  975.753949]  [<c02742fc>] fbcon_switch+0x16c/0x690
   [  975.753973]  [<c02cc645>] redraw_screen+0xe5/0x1c0
   [  975.754000]  [<c02c811a>] complete_change_console+0x2a/0xd0
   [  975.754027]  [<c02c765d>] vt_ioctl+0x103d/0x19b0
   [  975.754051]  [<c02c17eb>] tty_ioctl+0x18b/0x3d0
   [  975.754074]  [<c016e92a>] do_ioctl+0x5a/0x90
   [  975.754100]  [<c016eaeb>] vfs_ioctl+0x5b/0x1d0
   [  975.754127]  [<c016eca5>] sys_ioctl+0x45/0x70
   [  975.754153]  [<c01029a9>] syscall_call+0x7/0xb

Again a call to set_par:

   [  975.754744]  [<c0282331>] cyblafb_set_par+0x31/0xd80
   [  975.754773]  [<c0274759>] fbcon_switch+0x5c9/0x690
   [  975.754796]  [<c02cc645>] redraw_screen+0xe5/0x1c0
   [  975.754823]  [<c02c811a>] complete_change_console+0x2a/0xd0
   [  975.754850]  [<c02c765d>] vt_ioctl+0x103d/0x19b0
   [  975.754875]  [<c02c17eb>] tty_ioctl+0x18b/0x3d0
   [  975.754917]  [<c016e92a>] do_ioctl+0x5a/0x90
   [  975.754943]  [<c016eaeb>] vfs_ioctl+0x5b/0x1d0
   [  975.754970]  [<c016eca5>] sys_ioctl+0x45/0x70
   [  975.754996]  [<c01029a9>] syscall_call+0x7/0xb
   [  975.755032] cyblafb: Switching to new mode: fbset -g 1280 1024 
2048 4096 8 -t 7407 232 16 39 0 160 3
   [  975.757123] cyblafb: pixclock = 135.00 MHz, k/m/n 0 5 3a

Now the save_state() function is called:

   [  975.757745] cyblafb: Switching to KD_TEXT

and again a call to set_par():

   [  975.757759]  [<c0282331>] cyblafb_set_par+0x31/0xd80
   [  975.757789]  [<c027bb38>] fb_set_var+0x1e8/0x210
   [  975.757813]  [<c027498a>] fbcon_blank+0x11a/0x1b0
   [  975.757837]  [<c02cff17>] do_unblank_screen+0x67/0x120
   [  975.757869]  [<c02c8130>] complete_change_console+0x40/0xd0
   [  975.757955]  [<c02c765d>] vt_ioctl+0x103d/0x19b0
   [  975.757979]  [<c02c17eb>] tty_ioctl+0x18b/0x3d0
   [  975.758002]  [<c016e92a>] do_ioctl+0x5a/0x90
   [  975.758029]  [<c016eaeb>] vfs_ioctl+0x5b/0x1d0
   [  975.758055]  [<c016eca5>] sys_ioctl+0x45/0x70
   [  975.758081]  [<c01029a9>] syscall_call+0x7/0xb
   [  975.758117] cyblafb: Switching to new mode: fbset -g 1280 1024 
2048 4096 8 -t 7407 232 16 39 0 160 3
   [  975.760205] cyblafb: pixclock = 135.00 MHz, k/m/n 0 5 3a

And now, the be really shure, again, a call to set_par():

   [  975.760827]  [<c0282331>] cyblafb_set_par+0x31/0xd80
   [  975.760856]  [<c0274759>] fbcon_switch+0x5c9/0x690
   [  975.760880]  [<c02cc645>] redraw_screen+0xe5/0x1c0
   [  975.760923]  [<c02749e6>] fbcon_blank+0x176/0x1b0
   [  975.760947]  [<c02cff17>] do_unblank_screen+0x67/0x120
   [  975.760976]  [<c02c8130>] complete_change_console+0x40/0xd0
   [  975.761002]  [<c02c765d>] vt_ioctl+0x103d/0x19b0
   [  975.761027]  [<c02c17eb>] tty_ioctl+0x18b/0x3d0
   [  975.761050]  [<c016e92a>] do_ioctl+0x5a/0x90
   [  975.761076]  [<c016eaeb>] vfs_ioctl+0x5b/0x1d0
   [  975.761102]  [<c016eca5>] sys_ioctl+0x45/0x70
   [  975.761129]  [<c01029a9>] syscall_call+0x7/0xb
   [  975.761164] cyblafb: Switching to new mode: fbset -g 1280 1024 
2048 4096 8 -t 7407 232 16 39 0 160 3
   [  975.763280] cyblafb: pixclock = 135.00 MHz, k/m/n 0 5 3a

First of all: set_par is executed too often. With and without my patch.

If we would know that vt x is nothing but the vt used for X, we could 
introduce a policy
to completely ignore all framebuffer operations for that console. That 
also would result
in a much nicer look of the modeswitch. Do we need a new kernel 
parameter or should
this be implemented by detected which vt is switched to KD_GRAPHICS?

Switching from X to the console could be optimized a lot, I´m shure that 
you do agree.
We do need _one_ call to set_par() etc, but this must be at the right 
place. And the right
place is somewhere before any other driver operation is called, with the 
exception of
check_var() and init functions obviously.

Think about your NAK carefully. I might submit a patch to skeletonfb.c 
that inserts
a warning at all places that might be unexpectedly called with a 
hardware state different
to that set by set_par(). ;-))))

And please don´t argue that X or certain releases are broken. That is 
true, but ordinary
users will use those broken versions for years.

cu,
Knut
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