linux-kernel - Re: [Intel-gfx] [PATCH 01/13] drm/i915: fall through pwrite_gtt

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Date:	Mon, 21 Nov 2011 10:20:15 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Ben Widawsky <ben@...dawsk.net>,
	Daniel Vetter <daniel.vetter@...ll.ch>
Cc:	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH 01/13] drm/i915: fall through pwrite_gtt_slow to the shmem slow path

On Sun, 20 Nov 2011 19:09:18 -0800, Ben Widawsky <ben@...dawsk.net> wrote:
> On Sun,  6 Nov 2011 20:13:48 +0100
> Daniel Vetter <daniel.vetter@...ll.ch> wrote:
> 
> > The gtt_pwrite slowpath grabs the userspace memory with
> > get_user_pages. This will not work for non-page backed memory, like a
> > gtt mmapped gem object. Hence fall throuh to the shmem paths if we hit
> > -EFAULT in the gtt paths.
> > 
> > Now the shmem paths have exactly the same problem, but this way we
> > only need to rearrange the code in one write path.
> > 
> > v2: v1 accidentaly falls back to shmem pwrite for phys objects. Fixed.
> > 
> > Signed-Off-by: Daniel Vetter <daniel.vetter@...ll.ch>
> 
> It would be nice if there was some way to notify users that pwriting a
> gtt mmapped address can be really damn slow. That's also the one
> behavior change this patch introduces. It's possible that some SW was
> expecting to get a, "fast path" and would deal with the -EFAULT if it
> didn't get it.

The behaviour change is intentional. Before this patch we would
deadlock...
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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