linux-kernel - Re: [PATCH v2 1/2] iommu/arm-smmu-v3: Add support for ECMDQ register mode

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Date:   Thu, 10 Aug 2023 12:58:56 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     "thunder.leizhen@...weicloud.com" <thunder.leizhen@...weicloud.com>
CC:     Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
        "Joerg Roedel" <joro@...tes.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Tanmay Jagdale <tanmay@...vell.com>,
        "Jonathan Cameron" <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH v2 1/2] iommu/arm-smmu-v3: Add support for ECMDQ register
 mode

Hi Zhen,

On Wed, Aug 09, 2023 at 06:13:02AM -0700, thunder.leizhen@...weicloud.com wrote:
> +static int arm_smmu_ecmdq_layout(struct arm_smmu_device *smmu)
> +{
> +       int cpu;
> +       struct arm_smmu_ecmdq __percpu *ecmdq;
> +
> +       if (num_possible_cpus() <= smmu->nr_ecmdq) {

Does the nr_ecmdq always physically match with the number of CPUs?
I saw the spec saying "up to 256 pages", but not sure if they are
always physically present, even if CPU number is smaller i.e. some
of them would be unassigned/wasted.

> +               ecmdq = devm_alloc_percpu(smmu->dev, *ecmdq);
> +               if (!ecmdq)
> +                       return -ENOMEM;
> +
> +               for_each_possible_cpu(cpu)
> +                       *per_cpu_ptr(smmu->ecmdqs, cpu) = per_cpu_ptr(ecmdq, cpu);
> +
> +               /* A core requires at most one ECMDQ */
> +               smmu->nr_ecmdq = num_possible_cpus();
> +
> +               return 0;
> +       }
> +
> +       return -ENOSPC;

This ENOSPC is a software limitation, isn't it? How about using
"smp_processor_id() % smmu->nr_ecmdq" to select a queue?

> +       shift_increment = order_base_2(num_possible_cpus() / smmu->nr_ecmdq);
> +
> +       offset = 0;
> +       for_each_possible_cpu(cpu) {
> +               struct arm_smmu_ecmdq *ecmdq;
> +               struct arm_smmu_queue *q;
> +
> +               ecmdq = *per_cpu_ptr(smmu->ecmdqs, cpu);
> +               ecmdq->base = cp_base + offset;
> +
> +               q = &ecmdq->cmdq.q;
> +
> +               q->llq.max_n_shift = ECMDQ_MAX_SZ_SHIFT + shift_increment;
> +               ret = arm_smmu_init_one_queue(smmu, q, ecmdq->base, ARM_SMMU_ECMDQ_PROD,
> +                               ARM_SMMU_ECMDQ_CONS, CMDQ_ENT_DWORDS, "ecmdq");

Not getting why max_n_shift increases by shift_increment. Mind
elaborating?

Thanks
Nicolin