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

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Message-ID: <20200727145740.GA32521@jcrouse1-lnx.qualcomm.com>
Date:   Mon, 27 Jul 2020 08:57:41 -0600
From:   Jordan Crouse <jcrouse@...eaurora.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.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 Sun, Jul 26, 2020 at 11:27:03PM -0700, Bjorn Andersson wrote:
> 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);

Excellent suggestions.  Thanks.

Jordan

> Regards,
> Bjorn

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