Re: [PATCH] drm/fb-helper: Ignore the value of fb_var_screeninfo.pixclock

[Greg KH <greg@kroah.com>]

On Thu, Jan 17, 2019 at 08:02:33AM +0500, Ivan Mironov wrote:
> commit 66a8d5bfb518f9f12d47e1d2dce1732279f9451e upstream.
> 
> Backported by adjusting patch context.
> 
> Strict requirement of pixclock to be zero breaks support of SDL 1.2
> which contains hardcoded table of supported video modes with non-zero
> pixclock values[1].
> 
> To better understand which pixclock values are considered valid and how
> driver should handle these values, I briefly examined few existing fbdev
> drivers and documentation in Documentation/fb/. And it looks like there
> are no strict rules on that and actual behaviour varies:
> 
> 	* some drivers treat (pixclock == 0) as "use defaults" (uvesafb.c);
> 	* some treat (pixclock == 0) as invalid value which leads to
> 	  -EINVAL (clps711x-fb.c);
> 	* some pass converted pixclock value to hardware (uvesafb.c);
> 	* some are trying to find nearest value from predefined table
>           (vga16fb.c, video_gx.c).
> 
> Given this, I believe that it should be safe to just ignore this value if
> changing is not supported. It seems that any portable fbdev application
> which was not written only for one specific device working under one
> specific kernel version should not rely on any particular behaviour of
> pixclock anyway.
> 
> However, while enabling SDL1 applications to work out of the box when
> there is no /etc/fb.modes with valid settings, this change affects the
> video mode choosing logic in SDL. Depending on current screen
> resolution, contents of /etc/fb.modes and resolution requested by
> application, this may lead to user-visible difference (not always):
> image will be displayed in a right way, but it will be aligned to the
> left instead of center. There is no "right behaviour" here as well, as
> emulated fbdev, opposing to old fbdev drivers, simply ignores any
> requsts of video mode changes with resolutions smaller than current.
> 
> The easiest way to reproduce this problem is to install sdl-sopwith[2],
> remove /etc/fb.modes file if it exists, and then try to run sopwith
> from console without X. At least in Fedora 29, sopwith may be simply
> installed from standard repositories.
> 
> [1] SDL 1.2.15 source code, src/video/fbcon/SDL_fbvideo.c, vesa_timings
> [2] http://sdl-sopwith.sourceforge.net/
> 
> Signed-off-by: Ivan Mironov <mironov.ivan@gmail.com>
> Cc: stable@vger.kernel.org # 4.14.x
> Cc: stable@vger.kernel.org # 4.19.x
> Cc: stable@vger.kernel.org # 4.20.x
> Fixes: 79e539453b34e ("DRM: i915: add mode setting support")
> Fixes: 771fe6b912fca ("drm/radeon: introduce kernel modesetting for radeon hardware")
> Fixes: 785b93ef8c309 ("drm/kms: move driver specific fb common code to helper functions (v2)")
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Link: https://patchwork.freedesktop.org/patch/msgid/20190108072353.28078-3-mironov.ivan@gmail.com
> ---
>  drivers/gpu/drm/drm_fb_helper.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

All now queued up, thanks.

greg k-h