Re: [PATCH] iommu/arm-smmu-qcom: Fix fastrpc compatible string in ACTLR client match table

[Dmitry Baryshkov]

On Wed, Apr 08, 2026 at 06:38:25PM +0530, bibek.patro@xxxxxxxxxxxxxxxx wrote:
> From: Bibek Kumar Patro <bibek.patro@xxxxxxxxxxxxxxxx>
> 
> The qcom_smmu_actlr_client_of_match table contained "qcom,fastrpc" as
> the compatible string for applying ACTLR prefetch settings to FastRPC
> devices. However, "qcom,fastrpc" is the compatible string for the parent
> rpmsg channel node, which is not an IOMMU client — it carries no
> "iommus" property in the device tree and is never attached to an SMMU
> context bank.
> 
> The actual IOMMU clients are the compute context bank (CB) child nodes,
> which use the compatible string "qcom,fastrpc-compute-cb". These nodes
> carry the "iommus" property and are probed by fastrpc_cb_driver via
> fastrpc_cb_probe(), which sets up the DMA mask and IOMMU mappings for
> each FastRPC session. The device tree structure is:
> 
>   fastrpc {
>       compatible = "qcom,fastrpc";        /* rpmsg channel, no iommus */
>       ...
>       compute-cb@3 {
>           compatible = "qcom,fastrpc-compute-cb";
>           iommus = <&apps_smmu 0x1823 0x0>;  /* actual IOMMU client */
>       };
>   };
> 
> Since qcom_smmu_set_actlr_dev() calls of_match_device() against the
> device being attached to the SMMU context bank, the "qcom,fastrpc"
> entry was never matching any device. As a result, the ACTLR prefetch
> settings (PREFETCH_DEEP | CPRE | CMTLB) were silently never applied
> for FastRPC compute context banks.
> 
> Fix this by replacing "qcom,fastrpc" with "qcom,fastrpc-compute-cb"
> in the match table so that the ACTLR settings are correctly applied
> to the compute CB devices that are the true IOMMU clients.
> 
> Assisted-by: Anthropic:claude-4-6-sonnet
> Fixes: 3e35c3e725de ("iommu/arm-smmu: Add ACTLR data and support for qcom_smmu_500")
> Signed-off-by: Bibek Kumar Patro <bibek.patro@xxxxxxxxxxxxxxxx>
> ---
> 
> While there is an ongoing discussion [1] on how to differentiate ACTLR
> prefetch settings between compute DSP and audio DSP fastrpc devices, it
> is necessary to first fix the compatible string to "qcom,fastrpc-compute-cb".
> Both compute DSP and audio DSP fastrpc nodes use this compatible string,
> so both will receive the ACTLR settings after this fix. However, for
> audio DSP devices the effect remains the same as the current
> state since they do not actively use prefetch — the write is effectively
> a NOP for them. The fix is meaningful for compute DSP devices, which
> actively use prefetch and were previously being silently skipped.
> 
> [1]: https://lore.kernel.org/all/9b4c895a-c822-40e6-bb92-8fdcd09c82d3@xxxxxxxxxxxxxxxx/
> 
>  drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxxxxxxxx>


-- 
With best wishes
Dmitry