[PATCH] drm/i915/gt: Add a delay to let engine resumes correctly

[Andi Shyti]

Hi Nitin,

On Mon, Feb 24, 2025 at 12:01:04PM +0530, Nitin Gote wrote:
> Sometimes engine reset fails because the engine resumes from an
> incorrect RING_HEAD. Engine head failed to set to zero even after
> writing into it. This is a timing issue and we experimented
> different values and found out that 20ms delay works best based
> on testing.
> 
> So, add a 20ms delay to let engine resumes from correct RING_HEAD.
> 
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/13650
> Signed-off-by: Nitin Gote <nitin.r.gote at intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_ring_submission.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_ring_submission.c b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> index 6e9977b2d180..5576f000e965 100644
> --- a/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> +++ b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> @@ -365,6 +365,13 @@ static void reset_prepare(struct intel_engine_cs *engine)
>  			     ENGINE_READ_FW(engine, RING_HEAD),
>  			     ENGINE_READ_FW(engine, RING_TAIL),
>  			     ENGINE_READ_FW(engine, RING_START));
> +		/*
> +		 * Sometimes engine head failed to set to zero even after writing into it.
> +		 * Use 20ms delay to let engine resumes from correct RING_HEAD.
> +		 * Experimented different values and determined that 20ms works best
> +		 * based on testing.
> +		 */
> +		mdelay(20);

Is there any extremely strong reason for using mdelay here,
rather than any other delay function?

Andi

>  		if (!stop_ring(engine)) {
>  			drm_err(&engine->i915->drm,
>  				"failed to set %s head to zero "
> -- 
> 2.25.1