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Date:   Tue, 1 Aug 2023 14:55:08 +0100
From:   Will Deacon <will@kernel.org>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
Cc:     agross@kernel.org, andersson@kernel.org, luca@z3ntu.xyz,
        konrad.dybcio@linaro.org, dmitry.baryshkov@linaro.org,
        joro@8bytes.org, robin.murphy@arm.com, robh+dt@kernel.org,
        krzysztof.kozlowski+dt@linaro.org, conor+dt@kernel.org,
        robdclark@gmail.com, linux-arm-msm@vger.kernel.org,
        iommu@lists.linux.dev, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        kernel@collabora.com
Subject: Re: [PATCH v5 4/6] iommu/qcom: Index contexts by asid number to
 allow asid 0
Message-ID: <20230801135507.GB26253@willie-the-truck>
References: <20230622092742.74819-1-angelogioacchino.delregno@collabora.com>
 <20230622092742.74819-5-angelogioacchino.delregno@collabora.com>
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In-Reply-To: <20230622092742.74819-5-angelogioacchino.delregno@collabora.com>
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On Thu, Jun 22, 2023 at 11:27:40AM +0200, AngeloGioacchino Del Regno wrote:
> This driver was indexing the contexts by asid-1, which is probably
> done under the assumption that the first ASID is always 1.
> Unfortunately this is not always true: at least for MSM8956 and
> MSM8976's GPU IOMMU, the gpu_user context's ASID number is zero.
> To allow using a zero asid number, index the contexts by `asid`
> instead of by `asid - 1`.
> 
> While at it, also enhance human readability by renaming the
> `num_ctxs` member of struct qcom_iommu_dev to `max_asid`.
> 
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
> ---
>  drivers/iommu/arm/arm-smmu/qcom_iommu.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu/qcom_iommu.c b/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> index f1bd7c035db8..9786fd094e7d 100644
> --- a/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> +++ b/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> @@ -51,8 +51,8 @@ struct qcom_iommu_dev {
>  	struct clk_bulk_data clks[CLK_NUM];
>  	void __iomem		*local_base;
>  	u32			 sec_id;
> -	u8			 num_ctxs;
> -	struct qcom_iommu_ctx	*ctxs[];   /* indexed by asid-1 */
> +	u8			 max_asid;
> +	struct qcom_iommu_ctx	*ctxs[];   /* indexed by asid */
>  };
>  
>  struct qcom_iommu_ctx {
> @@ -94,7 +94,7 @@ static struct qcom_iommu_ctx * to_ctx(struct qcom_iommu_domain *d, unsigned asid
>  	struct qcom_iommu_dev *qcom_iommu = d->iommu;
>  	if (!qcom_iommu)
>  		return NULL;
> -	return qcom_iommu->ctxs[asid - 1];
> +	return qcom_iommu->ctxs[asid];
>  }
>  
>  static inline void
> @@ -534,12 +534,10 @@ static int qcom_iommu_of_xlate(struct device *dev, struct of_phandle_args *args)
>  	qcom_iommu = platform_get_drvdata(iommu_pdev);
>  
>  	/* make sure the asid specified in dt is valid, so we don't have
> -	 * to sanity check this elsewhere, since 'asid - 1' is used to
> -	 * index into qcom_iommu->ctxs:
> +	 * to sanity check this elsewhere:
>  	 */
> -	if (WARN_ON(asid < 1) ||
> -	    WARN_ON(asid > qcom_iommu->num_ctxs) ||
> -	    WARN_ON(qcom_iommu->ctxs[asid - 1] == NULL)) {
> +	if (WARN_ON(asid > qcom_iommu->max_asid) ||
> +	    WARN_ON(qcom_iommu->ctxs[asid] == NULL)) {
>  		put_device(&iommu_pdev->dev);
>  		return -EINVAL;
>  	}
> @@ -696,7 +694,7 @@ static int qcom_iommu_ctx_probe(struct platform_device *pdev)
>  
>  	dev_dbg(dev, "found asid %u\n", ctx->asid);
>  
> -	qcom_iommu->ctxs[ctx->asid - 1] = ctx;
> +	qcom_iommu->ctxs[ctx->asid] = ctx;
>  
>  	return 0;
>  }
> @@ -708,7 +706,7 @@ static void qcom_iommu_ctx_remove(struct platform_device *pdev)
>  
>  	platform_set_drvdata(pdev, NULL);
>  
> -	qcom_iommu->ctxs[ctx->asid - 1] = NULL;
> +	qcom_iommu->ctxs[ctx->asid] = NULL;
>  }
>  
>  static const struct of_device_id ctx_of_match[] = {
> @@ -755,11 +753,11 @@ static int qcom_iommu_device_probe(struct platform_device *pdev)
>  	for_each_child_of_node(dev->of_node, child)
>  		max_asid = max(max_asid, get_asid(child));
>  
> -	qcom_iommu = devm_kzalloc(dev, struct_size(qcom_iommu, ctxs, max_asid),
> +	qcom_iommu = devm_kzalloc(dev, struct_size(qcom_iommu, ctxs, max_asid + 1),
>  				  GFP_KERNEL);

So is this '+ 1' there to handle the case where ASIDs are indexed from 1? If
so, please add a comment because this isn't obvious at all.

Will