Re: [PATCH] drm/i915: Fix context size calculation on SNB/IVB/VLV

From: Ben Widawsky <ben@bwidawsk.net>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH] drm/i915: Fix context size calculation on SNB/IVB/VLV
Date: Thu, 22 Aug 2013 11:58:06 -0700	[thread overview]
Message-ID: <20130822185806.GC7181@bwidawsk.net> (raw)
In-Reply-To: <20130822185253.GF29682@intel.com>

On Thu, Aug 22, 2013 at 09:52:53PM +0300, Ville Syrjälä wrote:
> On Thu, Aug 22, 2013 at 11:30:55AM -0700, Ben Widawsky wrote:
> > On Thu, Aug 22, 2013 at 07:23:13PM +0300, ville.syrjala@linux.intel.com wrote:
> > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > 
> > > All the different context sizes reported in the CXT_SIZE register
> > > aren't meant to be simply added together.
> > > 
> > > While BSpec is somewhat unclear on the topic of the actual context
> > > size, empirical tests have now revealed the truth. So let's add a
> > > big fat comment to remind people how it all works.
> > 
> > By the way. I've done some digging. I believe (75% certain) pre-HSW,
> > every context save writes the entire data. So if you wanted to set some
> > pattern and see what HW actually overwrites, it should be doable. HSW+
> > though we can't do that.
> 
> Ah right, so you were thinking of initializing the context w/ some
> poison, and then see how much is left after the HW has saved. That I
> didnb't actually do. Should be reasonable trivial though, so I guess
> I'll try it. The same method could also be used to identify whether
> HSW skips parts or rearranges the context dynamically.

Yes, the only problem is it's not always trivial to invoke writes of
some parts of the context (I fear).

-- 
Ben Widawsky, Intel Open Source Technology Center
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