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

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Date:	Mon, 30 Jun 2014 14:39:48 +0200
From:	Thomas Hellstrom <thellstrom@...are.com>
To:	Christopher Friedt <chrisfriedt@...il.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 06/30/2014 02:25 PM, Christopher Friedt wrote:
> 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).

As I said. A kernel change that breaks existing user-space is ALWAYS BY
DEFINITION a kernel bug.
kernel changes are NOT ALLOWED to break existing user-space. The only
exception I can see here is if someone uses
the old non-kms driver which is not intended to work if vmware fbdev is
loaded but that's not the case here, from what I can tell?
>
>> 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.

xf86-video-vmware in kms mode uses the kernel driver to set these registers.
FWIW, the modesetting part of the kernel driver uses SVGA_REG_PITCHLOCK
instead of
SVGA_REG_BYTES_PER_LINE to set the pitch. That's probably where the
clash happens.

/Thomas


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