linux-kernel - Re: [PATCH] iommu/io-pgtable-arm: Allow non-coherent masters to use system cache

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Date:   Fri, 08 Jan 2021 11:17:25 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     isaacm@...eaurora.org
Cc:     Will Deacon <will@...nel.org>, Rob Clark <robdclark@...il.com>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, Joerg Roedel <joro@...tes.org>,
        linux-kernel@...r.kernel.org,
        Akhil P Oommen <akhilpo@...eaurora.org>,
        iommu@...ts.linux-foundation.org,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] iommu/io-pgtable-arm: Allow non-coherent masters to use
 system cache

On 2021-01-07 22:27, isaacm@...eaurora.org wrote:
> On 2021-01-06 03:56, Will Deacon wrote:
>> On Thu, Dec 24, 2020 at 12:10:07PM +0530, Sai Prakash Ranjan wrote:
>>> commit ecd7274fb4cd ("iommu: Remove unused IOMMU_SYS_CACHE_ONLY 
>>> flag")
>>> removed unused IOMMU_SYS_CACHE_ONLY prot flag and along with it went
>>> the memory type setting required for the non-coherent masters to use
>>> system cache. Now that system cache support for GPU is added, we will
>>> need to mark the memory as normal sys-cached for GPU to use system 
>>> cache.
>>> Without this, the system cache lines are not allocated for GPU. We 
>>> use
>>> the IO_PGTABLE_QUIRK_ARM_OUTER_WBWA quirk instead of a page 
>>> protection
>>> flag as the flag cannot be exposed via DMA api because of no in-tree
>>> users.
>>> 
>>> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
>>> ---
>>>  drivers/iommu/io-pgtable-arm.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>> 
>>> diff --git a/drivers/iommu/io-pgtable-arm.c 
>>> b/drivers/iommu/io-pgtable-arm.c
>>> index 7c9ea9d7874a..3fb7de8304a2 100644
>>> --- a/drivers/iommu/io-pgtable-arm.c
>>> +++ b/drivers/iommu/io-pgtable-arm.c
>>> @@ -415,6 +415,9 @@ static arm_lpae_iopte arm_lpae_prot_to_pte(struct 
>>> arm_lpae_io_pgtable *data,
>>>  		else if (prot & IOMMU_CACHE)
>>>  			pte |= (ARM_LPAE_MAIR_ATTR_IDX_CACHE
>>>  				<< ARM_LPAE_PTE_ATTRINDX_SHIFT);
>>> +		else if (data->iop.cfg.quirks & IO_PGTABLE_QUIRK_ARM_OUTER_WBWA)
>>> +			pte |= (ARM_LPAE_MAIR_ATTR_IDX_INC_OCACHE
>>> +				<< ARM_LPAE_PTE_ATTRINDX_SHIFT);
>>>  	}
>> 
> While this approach of enabling system cache globally for both page
> tables and other buffers
> works for the GPU usecase, this isn't ideal for other clients that use
> system cache. For example,
> video clients only want to cache a subset of their buffers in the
> system cache, due to the sizing constraint
> imposed by how much of the system cache they can use. So, it would be
> ideal to have
> a way of expressing the desire to use the system cache on a per-buffer
> basis. Additionally,
> our video clients use the DMA layer, and since the requirement is for
> caching in the system cache
> to be a per buffer attribute, it seems like we would have to have a
> DMA attribute to express
> this on a per-buffer basis.
> 

I did bring this up initially [1], also where is this video client
in upstream? AFAIK, only system cache user in upstream is GPU.
We cannot add any DMA attribute unless there is any user upstream
as per [2], so when the support for such a client is added, wouldn't
((data->iop.cfg.quirks & IO_PGTABLE_QUIRK_ARM_OUTER_WBWA) || PROT_FLAG)
work?

[1] 
https://lore.kernel.org/dri-devel/ecfda7ca80f6d7b4ff3d89b8758f4dc9@codeaurora.org/
[2] https://lore.kernel.org/linux-iommu/20191026053026.GA14545@lst.de/T/

Thanks,
Sai

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