[Intel-gfx] [benjamin.widawsky at intel.com: intel_gpu_top broken for HSW. Ideas needed]

[Daniel Vetter]

On Fri, Jul 12, 2013 at 7:12 PM, Ben Widawsky
<benjamin.widawsky at intel.com> wrote:
> FWD'd from our internal list now that we have more insight.
> ----- Forwarded message from Ben Widawsky <benjamin.widawsky at intel.com> -----
>
> Date: Thu, 11 Jul 2013 10:32:03 -0700
> From: Ben Widawsky <benjamin.widawsky at intel.com>
> To: linux-gfx at linux.intel.com
> Subject: intel_gpu_top broken for HSW. Ideas needed
> Message-ID: <20130711173202.GB8802 at intel.com>
>
> Hi everybody.
>
> While investigating a hard hang on Haswell. Eero noticed that
> intel_gpu_top helped to invoke the hang faster. I used this in my test
> case to validation, and they are suspecting it is a known issue which we
> have not yet worked around (and cannot reasonably workaround).
>
> [internal bug sighting redacted]
>
> To sum up, we cannot concurrently access registers within the same
> cacheline. It has the potential to hit a known bug.
>
> I see some choices:
> 1. Don't do anything.
> 2. Try to eliminate shared registers as much as possible. Instdone is
>    used by the hangcheck, and we can eliminate hangcheck with a
>    module parameter. Eero, can you try this as a workaround, btw?
> 3. Somehow make the kernel collect the top data and serialize access
>    there.
>
> Anyone else have input? I personally do not use top very much, so I
> won't be volunteering to do any of these.


For now I'd just vote for a warning on gen6+ on the intel-gpu-top
screen that this might hang hw. If anyone cares we could add a debugfs
interface (or finally get real approval for the performance counters
the hw has an expose them properly). Not a intel_gpu_top user myself
though.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch