linux-kernel - Re: [PATCH v2] iommu/arm-smmu-qcom: Rework the logic finding the bypass quirk

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Date:   Tue, 14 Mar 2023 16:56:20 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     will@...nel.org, joro@...tes.org, robin.murphy@....com,
        andersson@...nel.org, johan+linaro@...nel.org, steev@...i.org,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iommu/arm-smmu-qcom: Rework the logic finding the
 bypass quirk

On Tue, Mar 14, 2023 at 12:17:38PM +0100, Johan Hovold wrote:
> On Tue, Mar 14, 2023 at 04:29:05PM +0530, Manivannan Sadhasivam wrote:
> > The logic used to find the quirky firmware that intercepts the writes to
> > S2CR register to replace bypass type streams with a fault, and ignore the
> > fault type, is not working with the firmware on newer SoCs like SC8280XP.
> > 
> > The current logic uses the last stream mapping group (num_mapping_groups
> > - 1) as an index for finding quirky firmware. But on SC8280XP, NUSMRG
> > reports a value of 162 (possibly emulated by the hypervisor) and logic is
> > not working for stream mapping groups > 128. (Note that the ARM SMMU
> > architecture specification defines NUMSMRG in the range of 0-127).
> > 
> > So the current logic that checks the (162-1)th S2CR entry fails to detect
> > the quirky firmware on these devices and SMMU triggers invalid context
> > fault for bypass streams.
> > 
> > To fix this issue, rework the logic to find the first non-valid (free)
> > stream mapping register group (SMR) within 128 groups and use that index
> > to access S2CR for detecting the bypass quirk. If no free groups are
> > available, then just skip the quirk detection.
> > 
> > While at it, let's move the quirk detection logic to a separate function
> > and change the local variable name from last_s2cr to free_s2cr.
> > 
> > Reviewed-by: Bjorn Andersson <andersson@...nel.org>
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > ---
> > 
> > Changes in v2:
> > 
> > * Limited the check to 128 groups as per ARM SMMU spec's NUMSMRG range
> > * Moved the quirk handling to its own function
> > * Collected review tag from Bjorn
> > 
> >  drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 48 ++++++++++++++++++----
> >  1 file changed, 40 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > index d1b296b95c86..48362d7ef451 100644
> > --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > @@ -266,25 +266,49 @@ static int qcom_smmu_init_context(struct arm_smmu_domain *smmu_domain,
> >  	return 0;
> >  }
> >  
> > -static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu)
> > +static void qcom_smmu_bypass_quirk(struct arm_smmu_device *smmu)
> >  {
> > -	unsigned int last_s2cr = ARM_SMMU_GR0_S2CR(smmu->num_mapping_groups - 1);
> >  	struct qcom_smmu *qsmmu = to_qcom_smmu(smmu);
> > -	u32 reg;
> > -	u32 smr;
> > +	u32 free_s2cr;
> > +	u32 reg, smr;
> >  	int i;
> >  
> > +	/*
> > +	 * Find the first non-valid (free) stream mapping register group and
> > +	 * use that index to access S2CR for detecting the bypass quirk.
> > +	 *
> > +	 * Note that only the first 128 stream mapping groups are considered for
> > +	 * the check. This is because the ARM SMMU architecture specification
> > +	 * defines NUMSMRG (Number of Stream Mapping Register Groups) in the
> > +	 * range of 0-127, but some Qcom platforms emulate more stream mapping
> > +	 * groups with the help of hypervisor. And those groups don't exhibit
> > +	 * the quirky behavior.
> > +	 */
> > +	for (i = 0; i < 128; i++) {
> 
> This may now access registers beyond smmu->num_mapping_groups. Should
> you not use the minimum of these two values here (and below)?
> 

Doh! yeah, you're right. Will fix it in v3.

Thanks,
Mani

> > +		smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(i));
> > +
> > +		if (!FIELD_GET(ARM_SMMU_SMR_VALID, smr))
> > +			break;
> > +	}
> > +
> > +	/* If no free stream mapping register group is available, skip the check */
> > +	if (i == 128)
> > +		return;
> 
> Johan

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