linux-kernel - Re: [PATCH v10 04/13] iommu/arm-smmu-qcom: Add implementation for the adreno GPU SMMU

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Date:   Sun, 26 Jul 2020 23:27:03 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Jordan Crouse <jcrouse@...eaurora.org>
Cc:     linux-arm-msm@...r.kernel.org, Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>, freedreno@...ts.freedesktop.org,
        iommu@...ts.linux-foundation.org,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 04/13] iommu/arm-smmu-qcom: Add implementation for
 the adreno GPU SMMU

On Mon 20 Jul 08:40 PDT 2020, Jordan Crouse wrote:
> diff --git a/drivers/iommu/arm-smmu-qcom.c b/drivers/iommu/arm-smmu-qcom.c
[..]
> +static int qcom_adreno_smmu_alloc_context_bank(struct arm_smmu_domain *smmu_domain,
> +		struct device *dev, int start, int count)
> +{
> +	struct arm_smmu_device *smmu = smmu_domain->smmu;
> +
> +	/*
> +	 * Assign context bank 0 to the GPU device so the GPU hardware can
> +	 * switch pagetables
> +	 */
> +	if (qcom_adreno_smmu_is_gpu_device(dev)) {
> +		if (start > 0 || test_bit(0, smmu->context_map))
> +			return -ENOSPC;
> +
> +		set_bit(0, smmu->context_map);
> +		return 0;
> +	}
> +
> +	return __arm_smmu_alloc_bitmap(smmu->context_map, start, count);

If we end up here before the GPU device shows up this is going to
steal the first context bank, causing the subsequent allocation for the
GPU to always fail.

As such I think it would be appropriate for you to adjust "start" to
never be 0 here. And I think it would be appropriate to write this
function as:

	if (gpu) {
		start = 0;
		count = 1;
	} else {
		if (start == 0)
			start = 1;
	}

	return __arm_smmu_alloc_bitmap(smmu->context_map, start, count);

Regards,
Bjorn