[Mesa-dev] [PATCH] i965: Fix GPU hangs when a blorp batch is the first thing to execute.

[Eric Anholt]

Chris Wilson <chris at chris-wilson.co.uk> writes:

> On Thu, May 02, 2013 at 07:26:06AM -0700, Paul Berry wrote:
>>    Can you provide a documentation reference for why the value we're
>>    currently programming (0xfffff001) is unsafe, and why 0x7fff0001 is
>>    correct?� I don't see anything in the bspec.
>
> The largest GTT size for gen6 is 2GiB (it can be smaller on the whim of
> the BIOS, though we try to reset it back to 2GiB in i915.ko). The upper
> bound is used by the hardware to prevent invalid reads and return 0,
> this is the value we program to ~4GiB. The cause of these hangs is the
> constant data being read from addresses above 2GiB i.e. beyond the end
> of the GTT - and so prevented by programming the upper bound to the end
> of the GTT. Those with access to the simulator can hopefully verify
> this, and perhaps we should add this to the set of known bad commands in
> igt.

The simulator has no complaints about these batches.

I don't think your model of how the constants work (that there's some
undefined, possibly >2gb address being loaded at shader dispatch time,
which is fixed by sending these packets) is actually how they work --
From my reading, the constants are loaded at constant packet time into a
small buffer on the GPU.  If the last packet and the shader disagree
about that buffer, the GPU sometimes blows up.
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