Re: [PATCH 08/10] drm/panthor: Automatically enable interrupts in panthor_fw_wait_acks()

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

On 29/04/2026 10:38, Boris Brezillon wrote:
> Rather than assuming an interrupt is always expected for request
> acks, temporarily enable the relevant interrupts when the polling-wait
> failed. This should hopefully reduce the number of interrupts the CPU
> has to process.
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

It seems to work, although I'm lightly uneasy about this because I'm not
entirely sure whether the FW will immediately see the updates to
ack_irq_mask and therefore whether there's a possibility to miss an
event and be stuck waiting for the timeout.

Memory models are not my strong point, OpenAI tells me the sequence
should be something like:

  scoped_guard(spinlock_irqsave, lock) {
  	u32 ack_irq_mask = READ_ONCE(*ack_irq_mask_ptr);

  	WRITE_ONCE(*ack_irq_mask_ptr, ack_irq_mask | req_mask);
  }

  /*
   * The FW interface can be mapped write-combine/Normal-NC. Make sure the
   * IRQ mask update is visible to the FW before sleeping waiting for
the IRQ.
   */
  wmb();

Which seems plausible. But I've long ago learnt that plausible doesn't
mean much when dealing with memory models!

Thanks,
Steve

> ---
>  drivers/gpu/drm/panthor/panthor_fw.c    | 34 +++++++++++++++++++--------------
>  drivers/gpu/drm/panthor/panthor_sched.c |  5 +++--
>  2 files changed, 23 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_fw.c b/drivers/gpu/drm/panthor/panthor_fw.c
> index 8239a6951569..f5e0ceca4130 100644
> --- a/drivers/gpu/drm/panthor/panthor_fw.c
> +++ b/drivers/gpu/drm/panthor/panthor_fw.c
> @@ -1039,16 +1039,10 @@ static void panthor_fw_init_global_iface(struct panthor_device *ptdev)
>  	glb_iface->input->progress_timer = PROGRESS_TIMEOUT_CYCLES >> PROGRESS_TIMEOUT_SCALE_SHIFT;
>  	glb_iface->input->idle_timer = panthor_fw_conv_timeout(ptdev, IDLE_HYSTERESIS_US);
>  
> -	/* Enable interrupts we care about. */
> -	glb_iface->input->ack_irq_mask = GLB_CFG_ALLOC_EN |
> -					 GLB_PING |
> -					 GLB_CFG_PROGRESS_TIMER |
> -					 GLB_CFG_POWEROFF_TIMER |
> -					 GLB_IDLE_EN |
> -					 GLB_IDLE;
> -
> -	if (panthor_fw_has_glb_state(ptdev))
> -		glb_iface->input->ack_irq_mask |= GLB_STATE_MASK;
> +	/* Enable interrupts for asynchronous events that are not
> +	 * triggered by request acks.
> +	 */
> +	glb_iface->input->ack_irq_mask = GLB_IDLE;
>  
>  	panthor_fw_update_reqs(glb_iface, req, GLB_IDLE_EN | GLB_COUNTER_EN,
>  			       GLB_IDLE_EN | GLB_COUNTER_EN);
> @@ -1318,8 +1312,8 @@ void panthor_fw_unplug(struct panthor_device *ptdev)
>   * Return: 0 on success, -ETIMEDOUT otherwise.
>   */
>  static int panthor_fw_wait_acks(const u32 *req_ptr, const u32 *ack_ptr,
> -				wait_queue_head_t *wq,
> -				u32 req_mask, u32 *acked,
> +				u32 *ack_irq_mask_ptr, spinlock_t *lock,
> +				wait_queue_head_t *wq, u32 req_mask, u32 *acked,
>  				u32 timeout_ms)
>  {
>  	u32 ack, req = READ_ONCE(*req_ptr) & req_mask;
> @@ -1334,8 +1328,16 @@ static int panthor_fw_wait_acks(const u32 *req_ptr, const u32 *ack_ptr,
>  	if (!ret)
>  		return 0;
>  
> -	if (wait_event_timeout(*wq, (READ_ONCE(*ack_ptr) & req_mask) == req,
> -			       msecs_to_jiffies(timeout_ms)))
> +	scoped_guard(spinlock_irqsave, lock)
> +		*ack_irq_mask_ptr |= req_mask;
> +
> +	ret = wait_event_timeout(*wq, (READ_ONCE(*ack_ptr) & req_mask) == req,
> +				 msecs_to_jiffies(timeout_ms));
> +
> +	scoped_guard(spinlock_irqsave, lock)
> +		*ack_irq_mask_ptr &= ~req_mask;
> +
> +	if (ret)
>  		return 0;
>  
>  	/* Check one last time, in case we were not woken up for some reason. */
> @@ -1369,6 +1371,8 @@ int panthor_fw_glb_wait_acks(struct panthor_device *ptdev,
>  
>  	return panthor_fw_wait_acks(&glb_iface->input->req,
>  				    &glb_iface->output->ack,
> +				    &glb_iface->input->ack_irq_mask,
> +				    &glb_iface->lock,
>  				    &ptdev->fw->req_waitqueue,
>  				    req_mask, acked, timeout_ms);
>  }
> @@ -1395,6 +1399,8 @@ int panthor_fw_csg_wait_acks(struct panthor_device *ptdev, u32 csg_slot,
>  
>  	ret = panthor_fw_wait_acks(&csg_iface->input->req,
>  				   &csg_iface->output->ack,
> +				   &csg_iface->input->ack_irq_mask,
> +				   &csg_iface->lock,
>  				   &ptdev->fw->req_waitqueue,
>  				   req_mask, acked, timeout_ms);
>  
> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
> index 601a9bff1485..2edba335f22d 100644
> --- a/drivers/gpu/drm/panthor/panthor_sched.c
> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> @@ -1110,7 +1110,7 @@ cs_slot_prog_locked(struct panthor_device *ptdev, u32 csg_id, u32 cs_id)
>  	cs_iface->input->ringbuf_output = queue->iface.output_fw_va;
>  	cs_iface->input->config = CS_CONFIG_PRIORITY(queue->priority) |
>  				  CS_CONFIG_DOORBELL(queue->doorbell_id);
> -	cs_iface->input->ack_irq_mask = ~0;
> +	cs_iface->input->ack_irq_mask = CS_FATAL | CS_FAULT | CS_TILER_OOM;
>  	panthor_fw_update_reqs(cs_iface, req,
>  			       CS_IDLE_SYNC_WAIT |
>  			       CS_IDLE_EMPTY |
> @@ -1378,7 +1378,8 @@ csg_slot_prog_locked(struct panthor_device *ptdev, u32 csg_id, u32 priority)
>  		csg_iface->input->protm_suspend_buf = 0;
>  	}
>  
> -	csg_iface->input->ack_irq_mask = ~0;
> +	csg_iface->input->ack_irq_mask = CSG_SYNC_UPDATE | CSG_IDLE |
> +					 CSG_PROGRESS_TIMER_EVENT;
>  	panthor_fw_toggle_reqs(csg_iface, doorbell_req, doorbell_ack, queue_mask);
>  	return 0;
>  }
>