Re: [PATCH] drm/i915: fix forcewake related hangs on snb

[Chris Wilson <chris@chris-wilson.co.uk>]

On Thu, 26 Jul 2012 16:24:50 +0200, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> ... by adding seemingly redudant posting reads.
> 
> This little dragon lair exploded the first time around when we've
> refactored the code a bit to use the common wait_for_atomic_us in
> "drm/i915: Group the GT routines together in both code and vtable",
> which caused QA to file fdo bug #51738.
> 
> Chris Wilson entertained a few approaches to fixing #51738: Replacing
> the udelay(1) with the previously-used udelay(10) (or any other
> "sufficiently larger" delay), adding a posting read, or ditching the
> delay completely and using cpu_relax. We went with the cpu_relax and
> "915: Workaround hang with BSD and forcewake on SandyBridge". Which
> blew up in fdo bug #52424, but adding the posting read while still
> using cpu_relax seems to also fix that, it looks like the
> posting read is the important ingriedient to fix these rc6 related
> hangs on snb.
> 
> Popular theories as to why this is like it is include:
> - A herd of pink elephants got royally angered somehow.
> 
> - The gpu has internally different functional units and judging by the
>   register offsets, the forcewake request register and the forcewake
>   ack registers are _not_ in the same functional unit (or at least
>   aren't reached through the same routes). Hence the posting read
>   syncs up with the wrong block and gets the entire gpu confused.
> 
> - ...
> 
> As a minimal ducttape fix for 3.6, let's just put these posting reads
> into place again. We can try fancier approaches (like adding back the
> cpu_relax instead of the udelay) in -next.
> 
> This (re-)fixes a regression introduced in
> 
> commit 990bbdadabaa51828e475eda86ee5720a4910cc3
> Author: Chris Wilson <chris@chris-wilson.co.uk>
> Date:   Mon Jul 2 11:51:02 2012 -0300
> 
>     drm/i915: Group the GT routines together in both code and vtable
> 
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=52424
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51738u
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

No change on IVB, fixes the dummy_reloc_loop hang on SNB.

Tested-by: Chris Wilson <chris@chris-wilson.co.uk>

udelay() is winning the award for most popular function. Almost, it got
pipped at the last second by read_hpet() on earlier chipsets.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre