Re: [PATCH 5/9] drm/panfrost: Add HW_ISSUE_TTRX_3485 quirk

[Steven Price <steven.price@arm.com>]

On 11/02/2022 20:27, alyssa.rosenzweig@collabora.com wrote:
> From: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
> 
> TTRX_3485 requires the infamous "dummy job" workaround. I have this
> workaround implemented in a local branch, but I have not yet hit a case
> that requires it so I cannot test whether the implementation is correct.
> In the mean time, add the quirk bit so we can document which platforms
> may need it in the future.

This one is hideous ;) Although to me this isn't the 'infamous' one as
it's not the earliest example of a dummy job.

However... I believe as Panfrost currently stands this is probably not
very possible to hit. It requires a job to be stopped (soft or hard) at
a critical point during submission - which at the moment Panfrost
basically never does (the exception is if you close the fd immediately
while a job is in progress). And of course the timing has to be 'just
right' to hit the bug.

That said I think we should probably add pre-emption support sometime at
which point this could become an issue.

> Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

Reviewed-by: Steven Price <steven.price@arm.com>

> ---
>  drivers/gpu/drm/panfrost/panfrost_issues.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_issues.h b/drivers/gpu/drm/panfrost/panfrost_issues.h
> index 058f6a4c8435..b8865fc9efce 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_issues.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_issues.h
> @@ -132,6 +132,9 @@ enum panfrost_hw_issue {
>  	 * to hang */
>  	HW_ISSUE_TTRX_3076,
>  
> +	/* Must issue a dummy job before starting real work to prevent hangs */
> +	HW_ISSUE_TTRX_3485,
> +
>  	HW_ISSUE_END
>  };
>