Message-ID: <ac248f33-2528-c1d4-17ed-17e92e6ed5ad@arm.com>
Date:   Fri, 16 Aug 2019 17:58:24 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     freedreno@...ts.freedesktop.org, Rob Herring <robh@...nel.org>,
        Will Deacon <will@...nel.org>, jean-philippe.brucker@....com,
        linux-arm-msm@...r.kernel.org, Joerg Roedel <joro@...tes.org>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Zhen Lei <thunder.leizhen@...wei.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [Freedreno] [PATCH v3 0/2] iommu/arm-smmu: Split pagetable
 support

Hi Jordan,

On 15/08/2019 16:33, Jordan Crouse wrote:
> On Wed, Aug 07, 2019 at 04:21:38PM -0600, Jordan Crouse wrote:
>> (Sigh, resend. I freaked out my SMTP server)
>>
>> This is part of an ongoing evolution for enabling split pagetable support for
>> arm-smmu. Previous versions can be found [1].
>>
>> In the discussion for v2 Robin pointed out that this is a very Adreno specific
>> use case and that is exactly true. Not only do we want to configure and use a
>> pagetable in the TTBR1 space, we also want to configure the TTBR0 region but
>> not allocate a pagetable for it or touch it until the GPU hardware does so. As
>> much as I want it to be a generic concept it really isn't.
>>
>> This revision leans into that idea. Most of the same io-pgtable code is there
>> but now it is wrapped as an Adreno GPU specific format that is selected by the
>> compatible string in the arm-smmu device.
>>
>> Additionally, per Robin's suggestion we are skipping creating a TTBR0 pagetable
>> to save on wasted memory.
>>
>> This isn't as clean as I would like it to be but I think that this is a better
>> direction than trying to pretend that the generic format would work.
>>
>> I'm tempting fate by posting this and then taking some time off, but I wanted
>> to try to kick off a conversation or at least get some flames so I can try to
>> refine this again next week. Please take a look and give some advice on the
>> direction.
> 
> Will, Robin -
> 
> Modulo the impl changes from Robin, do you think that using a dedicated
> pagetable format is the right approach for supporting split pagetables for the
> Adreno GPU?

How many different Adreno drivers would benefit from sharing it?

The more I come back to this, the more I'm convinced that io-pgtable 
should focus on the heavy lifting of pagetable management - the code 
that nobody wants to have to write at all, let alone more than once - 
and any subtleties which aren't essential to that should be pushed back 
into whichever callers actually care. Consider that already, literally 
no caller actually uses an unmodified stage 1 TCR value as provided in 
the io_pgtable_cfg.

I feel it would be most productive to elaborate further in the form of 
patches, so let me get right on that and try to bash something out 
before I go home tonight...

Robin.

> If so, then is adding the changes to io-pgtable-arm.c possible for 5.4 and then
> add the implementation specific code on top of Robin's stack later or do you
> feel they should come as part of a package deal?
> 
> Jordan
> 
>> Jordan Crouse (2):
>>    iommu/io-pgtable-arm: Add support for ARM_ADRENO_GPU_LPAE io-pgtable
>>      format
>>    iommu/arm-smmu: Add support for Adreno GPU pagetable formats
>>
>>   drivers/iommu/arm-smmu.c       |   8 +-
>>   drivers/iommu/io-pgtable-arm.c | 214 ++++++++++++++++++++++++++++++++++++++---
>>   drivers/iommu/io-pgtable.c     |   1 +
>>   include/linux/io-pgtable.h     |   2 +
>>   4 files changed, 209 insertions(+), 16 deletions(-)
>>
>> -- 
>> 2.7.4
>>
>> _______________________________________________
>> Freedreno mailing list
>> Freedreno@...ts.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/freedreno
> 

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