linux-kernel - Re: [PATCH 1/2] dt-bindings: arm-smmu: Add qcom,last-ctx-bank-reserved

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Message-ID: <ZsPBeRTqE9o253Gw@hu-bjorande-lv.qualcomm.com>
Date: Mon, 19 Aug 2024 15:04:41 -0700
From: Bjorn Andersson <quic_bjorande@...cinc.com>
To: Marc Gonzalez <mgonzalez@...ebox.fr>
CC: Robin Murphy <robin.murphy@....com>, Rob Herring <robh@...nel.org>,
        Rob
 Clark <robdclark@...il.com>, Will Deacon <will@...nel.org>,
        Joerg Roedel
	<joro@...tes.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley
	<conor+dt@...nel.org>, <iommu@...ts.linux.dev>,
        <linux-arm-msm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <devicetree@...r.kernel.org>, Bjorn Andersson
	<andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Arnaud Vrac
	<avrac@...ebox.fr>,
        Pierre-Hugues Husson <phhusson@...ebox.fr>,
        Marijn
 Suijten <marijn.suijten@...ainline.org>,
        Caleb Connolly
	<caleb.connolly@...aro.org>
Subject: Re: [PATCH 1/2] dt-bindings: arm-smmu: Add
 qcom,last-ctx-bank-reserved

On Mon, Aug 19, 2024 at 05:02:16PM +0200, Marc Gonzalez wrote:
> On 19/08/2024 14:57, Robin Murphy wrote:
> 
> > Luckily, in this case it seems straightforward enough to be able to see 
> > that if we have a "qcom,msm8996-smmu-v2" with 13 context banks then we 
> > should just treat it as if it has 12 - it's also notable that it only 
> > reports NUMSMRG=12, so we couldn't use more than that many S1 context 
> > banks at once anyway.
> 
> This is what the hypervisor reports:
> 
> [    2.550974] arm-smmu 5100000.iommu: probing hardware configuration...
> [    2.557309] arm-smmu 5100000.iommu: SMMUv2 with:
> [    2.563815] arm-smmu 5100000.iommu:  stage 1 translation
> [    2.568494] arm-smmu 5100000.iommu:  address translation ops
> [    2.573791] arm-smmu 5100000.iommu:  non-coherent table walk
> [    2.579434] arm-smmu 5100000.iommu:  (IDR0.CTTW overridden by FW configuration)
> [    2.585088] arm-smmu 5100000.iommu:  stream matching with 12 register groups
> [    2.592132] arm-smmu 5100000.iommu:  13 context banks (0 stage-2 only)
> [    2.619316] arm-smmu 5100000.iommu:  Supported page sizes: 0x63315000
> [    2.626225] arm-smmu 5100000.iommu:  Stage-1: 36-bit VA -> 36-bit IPA
> [    2.632645] arm-smmu 5100000.iommu:  preserved 0 boot mappings
> 
> 
> smmu->num_mapping_groups = 12

Ignore num_mapping_groups, they are used to define which streams should
be mapped to which context bank. But there's no relationship between
these numbers.

> smmu->num_context_banks  = 13
> 
> 
> Are you saying that
> 
> 	smmu->num_context_banks > smmu->num_mapping_groups
> 
> does not make sense?
> 
> 
> Would a well-placed
> 
> 	if (smmu->num_context_banks > smmu->num_mapping_groups)
> 		smmu->num_context_banks = smmu->num_mapping_groups;
> 
> be a proper work-around?

No, something like this would apply your quirk to other targets (and
specifically it would be wrong, per above).

> 
> (Probably in qcom_smmu_cfg_probe() so as to not interfere with other platforms.)
> 
> 
> Maybe to limit the side effects even more:
> 
> 	if (of_device_is_compatible(smmu->dev->of_node, "qcom,msm8998-smmu-v2") &&
> 		smmu->num_context_banks > smmu->num_mapping_groups))
> 		smmu->num_context_banks = smmu->num_mapping_groups;

If we don't want to introduce a more specific compatible for this SMMU
instance, then let's add this to qcom_smmu_cfg_probe():

	/* MSM8998 LPASS SMMU reports 13 context banks, but only 12 are accessible */
 	if (of_device_is_compatible(smmu->dev->of_node, "qcom,msm8998-smmu-v2") && smmu->num_context_banks == 13)
		smmu->num_context_banks = 12;


Regards,
Bjorn

> 
> 
> Neither work-around would require changing the binding.
> 
> Is either work-around acceptable, Robin?
> 
> Regards
> 
>