linux-kernel - Re: [PATCH 1/1] drm/vmwgfx: correct fb_fix_screeninfo.line

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Message-ID: <CAF4BF-T4VZQtd_3wWfNQ_==KJE0z0N5XonG4jcy=Ys5x_XxJ=w@mail.gmail.com>
Date:	Mon, 30 Jun 2014 08:25:40 -0400
From:	Christopher Friedt <chrisfriedt@...il.com>
To:	Thomas Hellstrom <thellstrom@...are.com>
Cc:	dri-devel <dri-devel@...ts.freedesktop.org>,
	linux-kernel@...r.kernel.org, Dave Airlie <airlied@...il.com>,
	linux-graphics-maintainer@...are.com
Subject: Re: [PATCH 1/1] drm/vmwgfx: correct fb_fix_screeninfo.line_length

On Mon, Jun 30, 2014 at 7:48 AM, Thomas Hellstrom <thellstrom@...are.com> wrote:
> I don't think we can blame video-vmware for this. A kernel driver change
> that breaks existing user-space is by definition a kernel driver bug,
> regardless whether exisiting user-space is doing something horrendously
> stupid.

I wouldn't be so quick to say it's a kernel bug. The fbdev contract
hasn't changed. Also xf86-video-vmware isn't using the fbdev driver,
and the fbdev driver code is obviously correct (see screenshots in
link submitted with initial patch).

> So the fix must IMO be a kernel driver fix. My initial guess is that
> once we set the bytes per line register, it might not be automatically
> updated when the screen width is changed, but the documentation is poor.
> I see if I can shed some light over this.

Having dumped all of the svga registers while hacking on vmwgfx, I
noticed that the BYTES_PER_LINE field is initially incorrectly set to
something way off. My initial reaction is that video-vmware doesn't
properly compute the bytes-per-line register, and therefore that it is
a video-vmware bug that has always existed.

I'm reproducing the problem and providing a fix for video-vmware as I
write this.

C
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