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

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Date:   Mon, 11 Jan 2021 10:26:32 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Will Deacon <will@...nel.org>
Cc:     isaacm@...eaurora.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 2021-01-08 23:48, Will Deacon wrote:
> On Fri, Jan 08, 2021 at 11:17:25AM +0530, Sai Prakash Ranjan wrote:
>> 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?
> 
> Hmm, I think this is another case where we need to separate out the
> page-table walker attributes from the access attributes. Currently,
> IO_PGTABLE_QUIRK_ARM_OUTER_WBWA applies _only_ to the page-table walker
> and I don't think it makes any sense for that to be per-buffer (how 
> would
> you even manage that?). However, if we want to extend this to data 
> accesses
> and we know that there are valid use-cases where this should be 
> per-buffer,
> then shoe-horning it in with the walker quirk does not feel like the 
> best
> thing to do.
> 
> As a starting point, we could:
> 
>   1. Rename IO_PGTABLE_QUIRK_ARM_OUTER_WBWA to IO_PGTABLE_QUIRK_PTW_LLC
>   2. Add a new prot flag IOMMU_LLC
>   3. Have the GPU pass the new prot for its buffer mappings
> 

This looks good to me, I will work on this and post something soon.

> Does that work? One thing I'm not sure about is whether IOMMU_CACHE 
> should
> imply IOMMU_LLC, or whether there is a use-case for inner-cacheable, 
> outer
> non-cacheable mappings for a coherent device. Have you ever seen that 
> sort
> of thing before?
> 

I don't think there is such a usecase as Isaac mentioned.

Thanks,
Sai

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