Re: [PATCH v1 2/2] drm/fb-helper: Ignore the value of fb_var_screeninfo.pixclock

[Daniel Vetter]

On Fri, Dec 28, 2018 at 04:13:08AM +0500, Ivan Mironov wrote:
> 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.
> 
> Feel free to NAK this patch if you think that it causes breakage of
> user-space =).

It's a regression, we don't nack regression fixes :-)

> 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@xxxxxxxxx>

I thought this is also a regression fix, so also needs Fixes: and cc:
stable?
-Daniel

> ---
>  drivers/gpu/drm/drm_fb_helper.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index aff576c3c4fb..b95a0c23c7c8 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -1690,9 +1690,14 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
>  	struct drm_fb_helper *fb_helper = info->par;
>  	struct drm_framebuffer *fb = fb_helper->fb;
>  
> -	if (var->pixclock != 0 || in_dbg_master())
> +	if (in_dbg_master())
>  		return -EINVAL;
>  
> +	if (var->pixclock != 0) {
> +		DRM_DEBUG("fbdev emulation doesn't support changing the pixel clock, value of pixclock is ignored\n");
> +		var->pixclock = 0;
> +	}
> +
>  	if ((drm_format_info_block_width(fb->format, 0) > 1) ||
>  	    (drm_format_info_block_height(fb->format, 0) > 1))
>  		return -EINVAL;
> -- 
> 2.20.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch