Re: [PATCH v1] drm/xe/ct: Separate waiting for retry from ct send function

[Matthew Brost <matthew.brost@intel.com>]

On Thu, Oct 09, 2025 at 07:08:44PM +0200, Tomasz Lis wrote:
> The function `guc_ct_send_locked()` is really quite simple, but still
> looks complex due to exposed internals. It is sending a message,
> and in case of lack of space, waiting for a proper moment to send
> a retry.
> 
> Clear separation of send function and wait function will help with
> readability. This is a cosmetic change only, no functional difference
> is expected.
> 
> This patch introduces `guc_ct_send_wait_for_retry()`, and uses it to
> greatly simplify `guc_ct_send_locked()`.
> 
> Signed-off-by: Tomasz Lis <tomasz.lis@intel.com>

Reviewed-by: Matthew Brost <matthew.brost@intel.com>

> ---
>  drivers/gpu/drm/xe/xe_guc_ct.c | 64 +++++++++++++++++++++-------------
>  1 file changed, 39 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_guc_ct.c b/drivers/gpu/drm/xe/xe_guc_ct.c
> index 3ae1e8db143a..e68953ef3a00 100644
> --- a/drivers/gpu/drm/xe/xe_guc_ct.c
> +++ b/drivers/gpu/drm/xe/xe_guc_ct.c
> @@ -991,22 +991,15 @@ static void kick_reset(struct xe_guc_ct *ct)
>  
>  static int dequeue_one_g2h(struct xe_guc_ct *ct);
>  
> -static int guc_ct_send_locked(struct xe_guc_ct *ct, const u32 *action, u32 len,
> -			      u32 g2h_len, u32 num_g2h,
> -			      struct g2h_fence *g2h_fence)
> +/*
> + * wait before retry of sending h2g message
> + * Return: true if ready for retry, false if the wait timeouted
> + */
> +static bool guc_ct_send_wait_for_retry(struct xe_guc_ct *ct, u32 len,
> +				       u32 g2h_len, struct g2h_fence *g2h_fence,
> +				       unsigned int *sleep_period_ms)
>  {
>  	struct xe_device *xe = ct_to_xe(ct);
> -	struct xe_gt *gt = ct_to_gt(ct);
> -	unsigned int sleep_period_ms = 1;
> -	int ret;
> -
> -	xe_gt_assert(gt, !g2h_len || !g2h_fence);
> -	lockdep_assert_held(&ct->lock);
> -	xe_device_assert_mem_access(ct_to_xe(ct));
> -
> -try_again:
> -	ret = __guc_ct_send_locked(ct, action, len, g2h_len, num_g2h,
> -				   g2h_fence);
>  
>  	/*
>  	 * We wait to try to restore credits for about 1 second before bailing.
> @@ -1015,24 +1008,22 @@ static int guc_ct_send_locked(struct xe_guc_ct *ct, const u32 *action, u32 len,
>  	 * the case of G2H we process any G2H in the channel, hopefully freeing
>  	 * credits as we consume the G2H messages.
>  	 */
> -	if (unlikely(ret == -EBUSY &&
> -		     !h2g_has_room(ct, len + GUC_CTB_HDR_LEN))) {
> +	if (!h2g_has_room(ct, len + GUC_CTB_HDR_LEN)) {
>  		struct guc_ctb *h2g = &ct->ctbs.h2g;
>  
> -		if (sleep_period_ms == 1024)
> -			goto broken;
> +		if (*sleep_period_ms == 1024)
> +			return false;
>  
>  		trace_xe_guc_ct_h2g_flow_control(xe, h2g->info.head, h2g->info.tail,
>  						 h2g->info.size,
>  						 h2g->info.space,
>  						 len + GUC_CTB_HDR_LEN);
> -		msleep(sleep_period_ms);
> -		sleep_period_ms <<= 1;
> -
> -		goto try_again;
> -	} else if (unlikely(ret == -EBUSY)) {
> +		msleep(*sleep_period_ms);
> +		*sleep_period_ms <<= 1;
> +	} else {
>  		struct xe_device *xe = ct_to_xe(ct);
>  		struct guc_ctb *g2h = &ct->ctbs.g2h;
> +		int ret;
>  
>  		trace_xe_guc_ct_g2h_flow_control(xe, g2h->info.head,
>  						 desc_read(xe, g2h, tail),
> @@ -1046,7 +1037,7 @@ static int guc_ct_send_locked(struct xe_guc_ct *ct, const u32 *action, u32 len,
>  	(desc_read(ct_to_xe(ct), (&ct->ctbs.g2h), tail) != ct->ctbs.g2h.info.head)
>  		if (!wait_event_timeout(ct->wq, !ct->g2h_outstanding ||
>  					g2h_avail(ct), HZ))
> -			goto broken;
> +			return false;
>  #undef g2h_avail
>  
>  		ret = dequeue_one_g2h(ct);
> @@ -1054,9 +1045,32 @@ static int guc_ct_send_locked(struct xe_guc_ct *ct, const u32 *action, u32 len,
>  			if (ret != -ECANCELED)
>  				xe_gt_err(ct_to_gt(ct), "CTB receive failed (%pe)",
>  					  ERR_PTR(ret));
> -			goto broken;
> +			return false;
>  		}
> +	}
> +	return true;
> +}
> +
> +static int guc_ct_send_locked(struct xe_guc_ct *ct, const u32 *action, u32 len,
> +			      u32 g2h_len, u32 num_g2h,
> +			      struct g2h_fence *g2h_fence)
> +{
> +	struct xe_gt *gt = ct_to_gt(ct);
> +	unsigned int sleep_period_ms = 1;
> +	int ret;
> +
> +	xe_gt_assert(gt, !g2h_len || !g2h_fence);
> +	lockdep_assert_held(&ct->lock);
> +	xe_device_assert_mem_access(ct_to_xe(ct));
>  
> +try_again:
> +	ret = __guc_ct_send_locked(ct, action, len, g2h_len, num_g2h,
> +				   g2h_fence);
> +
> +	if (unlikely(ret == -EBUSY)) {
> +		if (!guc_ct_send_wait_for_retry(ct, len, g2h_len, g2h_fence,
> +						&sleep_period_ms))
> +			goto broken;
>  		goto try_again;
>  	}
>  
> -- 
> 2.25.1
>