Re: [PATCH v3 1/2] drm/i915: add render state initialization

[Damien Lespiau <damien.lespiau@intel.com>]

On Wed, May 14, 2014 at 10:24:53AM +0000, Mateo Lozano, Oscar wrote:
> Hi Mika,
> 
> > -----Original Message-----
> > From: Intel-gfx [mailto:intel-gfx-bounces@lists.freedesktop.org] On Behalf Of
> > Mika Kuoppala
> > Sent: Tuesday, May 06, 2014 3:30 PM
> > To: intel-gfx@lists.freedesktop.org
> > Cc: ben@bwidawsk.net; miku@iki.fi; kristen@linux.intel.com
> > Subject: [Intel-gfx] [PATCH v3 1/2] drm/i915: add render state initialization
> > 
> > HW guys say that it is not a cool idea to let device go into rc6 without proper 3d
> > pipeline state.
> > 
> > For each new uninitialized context, generate a valid null render state to be run
> > on context creation.
> 
> In Android, we have been seeing a problem in BDW D0 stepping (C0 is
> fine), in which actual rendering does not happen, even though
> everything seems to be healthy. The only "tell" seems to be that the
> pixel shader invocation count does not go up.  I wouldn´t dare say I
> understand why this fixes our problem, but it clearly does, so feel
> free to add:
> 
> Tested-by: Oscar Mateo <oscar.mateo@intel.com>

Stating the obvious here, mainly for my own understanding :)

FWIW, that looks like the userspace driver you are using actually
relying on the kernel setting up a golden state and missing "one bit" or
one packet. For instance it could be that it's missing a
3DSTATE_WM_HZ_OP command (that's the last fix Ken did in the gen8 render
copy state).

Out of sheer luck (or almost :) we happen to have this working.

So that's another kind of papering over than the one "needed" for rc6.

It seems to potentially be a useful service the kernel is providing to
user space apps though, trying to setup a sane state so user space
batches don't hang the GPU if they are missing one command. Of course
hangs can still happen if the batches themselves have bugs.

How to be sure that's the correct golden state is another interesting
question we need to answer.

-- 
Damien