linux-kernel - Re: [PATCH v1 2/2] drm/fb-helper: Ignore the value of fb_var

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Date:   Sat, 05 Jan 2019 21:21:21 +0500
From:   Ivan Mironov <mironov.ivan@...il.com>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        saahriktu <mail@...hriktu.org>,
        Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
Subject: Re: [PATCH v1 2/2] drm/fb-helper: Ignore the value of
 fb_var_screeninfo.pixclock

On Fri, 2018-12-28 at 13:06 +0100, Daniel Vetter wrote:
> 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@...il.com>
> 
> I thought this is also a regression fix, so also needs Fixes: and cc:
> stable?
> -Daniel

This particular patch fixes a bug that existed for 10 years unnoticed.
Are "Fixes:" and "Cc: stable" really required?

"pixclock != 0" check was introduced in this file by 785b93ef8c309
("drm/kms: move driver specific fb common code to helper functions
(v2)"). But actually similar code existed in the device-specific
drivers even earlier.

> 
> > ---
> >  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
> >