Re: [RFC PATCH 2/5] iommu/arm-smmu-v3: Add a helper to wait till cmdq drains

[Mostafa Saleh <smostafa@google.com>]

On Wed, Mar 19, 2025 at 12:42:51AM +0000, Pranjal Shrivastava wrote:
> Before we suspend SMMU, we want to ensure that all commands have been
> consumed by the SMMU's command queue, i.e. the cmdq is empty. Add a
> helper function that waits till the cmdq is empty using `queue_empty`.
> 
> Signed-off-by: Pranjal Shrivastava <praan@google.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 24 +++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 80362d9f293b..f65d9bca0392 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -754,6 +754,30 @@ static int arm_smmu_cmdq_poll_until_sync(struct arm_smmu_device *smmu,
>  	return __arm_smmu_cmdq_poll_until_consumed(smmu, cmdq, llq);
>  }
>  
> +/*
> + * Wait until the SMMU cmdq is empty
> + * Must be called with the cmdq lock held in some capacity.
> + */
> +static int arm_smmu_cmdq_poll_until_empty(struct arm_smmu_device *smmu,
> +					  struct arm_smmu_cmdq *cmdq,
> +					  struct arm_smmu_ll_queue *llq)
> +{
> +	struct arm_smmu_queue_poll qp;
> +	int ret = 0;
> +
> +	queue_poll_init(smmu, &qp);
> +	llq->val = READ_ONCE(cmdq->q.llq.val);
> +	do {
> +		if (queue_empty(llq))
> +			break;
> +
> +		ret = queue_poll(&qp);
> +		llq->cons = readl(cmdq->q.cons_reg);

If the command queue is not empty, that means some other thread is already
polling it, so I guess there is no need to do any MMIO here, just poll
the SW view of prod/cons, something similar to arm_smmu_cmdq_poll_until_not_full()

Thanks,
Mostafa


> +	} while (!ret);
> +
> +	return ret;
> +}
> +
>  static void arm_smmu_cmdq_write_entries(struct arm_smmu_cmdq *cmdq, u64 *cmds,
>  					u32 prod, int n)
>  {
> -- 
> 2.49.0.rc1.451.g8f38331e32-goog
>