Re: [PATCH 1/2] lib/gt: Omit illegal instruction on hang injection with gen 8+

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 1/2] lib/gt: Omit illegal instruction on hang injection with gen 8+
Date: Wed, 8 Jun 2016 14:54:53 +0100	[thread overview]
Message-ID: <20160608135453.GG32344@nuc-i3427.alporthouse.com> (raw)
In-Reply-To: <1465391238-32737-1-git-send-email-mika.kuoppala@intel.com>

On Wed, Jun 08, 2016 at 04:07:17PM +0300, Mika Kuoppala wrote:
> 0xffffffff as an illegal command confuses the command execution
> on gen9 so that the next BB start will get ignored, causing a runaway
> head past the bb end. This delays the hang detection substantially
> as hangcheck then observes only a non progressing seqno.
> 
> Omit the bad instruction on gen8+ and rely on the chained batch
> loop to cause a deterministic hang. Make the chained bb start
> to jump straight into bb start, omitting the MI_NOOP or the bad
> instruction on subsequent passes. This makes the acthd sampling
> to hit more reliably to the same value, as the loop is smaller,
> making the head appear to be 'more stuck'.

You could note that BB(addr | 4) is also illegal. gen2+ requires 8 byte
alignment, ilk requires 64 byte.

> References: https://bugs.freedesktop.org/show_bug.cgi?id=92715
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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