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

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Date:   Mon, 7 Jan 2019 11:12:55 +0100
From:   Greg KH <greg@...ah.com>
To:     Ivan Mironov <mironov.ivan@...il.com>
Cc:     Daniel Vetter <daniel@...ll.ch>, 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 Sat, Jan 05, 2019 at 09:21:21PM +0500, Ivan Mironov wrote:
> 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?

If you want it backported into a stable kernel release, then yes, they
are needed :)

thanks,

greg k-h