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:

quoted

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