linux-kernel - Re: [PATCH] iommu/arm-smmu-qcom: Work around SDM845 Adreno SMMU w/ 16K pages

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Message-ID: <20240823152147.GA396@willie-the-truck>
Date: Fri, 23 Aug 2024 16:21:48 +0100
From: Will Deacon <will@...nel.org>
To: Trilok Soni <quic_tsoni@...cinc.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	Konrad Dybcio <konradybcio@...il.com>,
	Rob Clark <robdclark@...il.com>,
	Konrad Dybcio <konradybcio@...nel.org>,
	Bjorn Andersson <andersson@...nel.org>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	iommu@...ts.linux.dev, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [PATCH] iommu/arm-smmu-qcom: Work around SDM845 Adreno SMMU w/
 16K pages

On Tue, Jul 30, 2024 at 10:16:00AM -0700, Trilok Soni wrote:
> On 7/30/2024 1:50 AM, Dmitry Baryshkov wrote:
> >>>>>> SDM845's Adreno SMMU is unique in that it actually advertizes support
> >>>>>> for 16K (and 32M) pages, which doesn't hold for newer SoCs.
> > My question is about forbidding 16k pages for sdm845 only or for other
> > chips too. I'd assume that it shouldn't also work for other smmu-v2
> > platforms.
> 
> Yes, my understanding was that SMMUv2 based IPs doesn't have 16k support
> and it is only starting from SMMUv3. 

I'm not sure about that. The architecture doc for SMMUv2 talks about the
AArch64 translation regime in section 1.5 and bit 13 of SMMU_IDR2 says:

  | PTFSv8_16kB, bit[13]
  | Support for 16KB translation granule size. The possible values of this bit are:
  | 0	The 16KB translation granule is not supported.
  | 1	The 16KB translation granule is supported.
  | In SMMUv1, this bit is reserved.

so I think Konrad's patch is about right, but if you want to extend it
to cover other implementations then that's fine too.

Will