Re: [PATCH] drm/i915: Add support for resetting the SO write pointers on gen7.

From: Eric Anholt <eric@anholt.net>
To: Eugeni Dodonov <eugeni@dodonov.net>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH] drm/i915: Add support for resetting the SO write pointers on gen7.
Date: Tue, 03 Jan 2012 17:56:09 -0800	[thread overview]
Message-ID: <87hb0cmds6.fsf@eliezer.anholt.net> (raw)
In-Reply-To: <CAC7LmnsduLA3DL67BgndUsZCE9ojdtyapqt2W9ARkY7tDyEy5A@mail.gmail.com>

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On Mon, 2 Jan 2012 13:04:37 -0200, Eugeni Dodonov <eugeni@dodonov.net> wrote:
> On Thu, Dec 29, 2011 at 23:52, Eric Anholt <eric@anholt.net> wrote:
> 
> > These registers are automatically incremented by the hardware during
> > transform feedback to track where the next streamed vertex output
> > should go.  Unlike the previous generation, which had a packet for
> > setting the corresponding registers to a defined value, gen7 only has
> > MI_LOAD_REGISTER_IMM to do so.  That's a secure packet (since it loads
> > an arbitrary register), so we need to do it from the kernel, and it
> > needs to be settable atomically with the batchbuffer execution so that
> > two clients doing transform feedback don't stomp on each others'
> > state.
> >
> > Instead of building a more complicated interface involcing setting the
> > registers to a specific value, just set them to 0 when asked and
> > userland can tweak its pointers accordingly.

> > diff --git a/drivers/gpu/drm/i915/i915_dma.c
> > b/drivers/gpu/drm/i915/i915_dma.c
> > index a9ae374..1add685 100644
> > --- a/drivers/gpu/drm/i915/i915_dma.c
> > +++ b/drivers/gpu/drm/i915/i915_dma.c
> > @@ -781,6 +781,9 @@ static int i915_getparam(struct drm_device *dev, void
> > *data,
> >        case I915_PARAM_HAS_RELAXED_DELTA:
> >                value = 1;
> >                break;
> > +       case I915_PARAM_HAS_GEN7_SOL_RESET:
> > +               value = 1;
> >
> 
> Wouldn't it be better to have:
>                    value = IS_GEN7(dev);
> 
> as it is gen7+-specific item. This way, userspace could check for this
> support early, and avoid setting the flag on the batchbuffer in vain on
> pre-gen7 architectures.
> 
> Either way, it will work, so:
> 
> Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>

The flag only gets set by userland in the gen7 code, so it wouldn't
change anything.

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