Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Christoph Hellwig]

On Mon, Oct 05, 2020 at 10:35:43AM -0400, Jonathan Marek wrote:
> The cache synchronization doesn't have anything to do with IOMMU (for
> example: cache synchronization would be useful in cases where drm/msm
> doesn't use IOMMU).

It has to do with doing DMA.  And we have two frameworks for doing DMA:
either the DMA API which is for general driver use, and which as part of
the design includes cache maintainance hidden behind the concept of
ownership transfers.  And we have the much more bare bones IOMMU API.

If people want to use the "raw" IOMMU API with not cache coherent
devices we'll need a cache maintainance API that goes along with it.
It could either be formally part of the IOMMU API or be separate.

> What is needed is to call arch_sync_dma_for_{cpu,device} (which is what I
> went with initially, but then decided to re-use drm/msm's
> sync_for_{cpu,device}). But you are also saying those functions aren't for
> driver use, and I doubt IOMMU maintainers will want to add wrappers for
> these functions just to satisfy this "not for driver use" requirement.

arch_sync_dma_for_{cpu,device} are low-level helpers (and not very
great ones at that).  The definitively should not be used by drivers.
They would be very useful buildblocks for a IOMMU cache maintainance
API.

Of course the best outcome would be if we could find a way for the MSM
drm driver to just use DMA API and not deal with the lower level
abstractions.  Do you remember why the driver went for use of the IOMMU
API?
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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Jonathan Marek]

On 10/6/20 3:23 AM, Christoph Hellwig wrote:
> On Mon, Oct 05, 2020 at 10:35:43AM -0400, Jonathan Marek wrote:
>> The cache synchronization doesn't have anything to do with IOMMU (for
>> example: cache synchronization would be useful in cases where drm/msm
>> doesn't use IOMMU).
> 
> It has to do with doing DMA.  And we have two frameworks for doing DMA:
> either the DMA API which is for general driver use, and which as part of
> the design includes cache maintainance hidden behind the concept of
> ownership transfers.  And we have the much more bare bones IOMMU API.
> 
> If people want to use the "raw" IOMMU API with not cache coherent
> devices we'll need a cache maintainance API that goes along with it.
> It could either be formally part of the IOMMU API or be separate.
> 
>> What is needed is to call arch_sync_dma_for_{cpu,device} (which is what I
>> went with initially, but then decided to re-use drm/msm's
>> sync_for_{cpu,device}). But you are also saying those functions aren't for
>> driver use, and I doubt IOMMU maintainers will want to add wrappers for
>> these functions just to satisfy this "not for driver use" requirement.
> 
> arch_sync_dma_for_{cpu,device} are low-level helpers (and not very
> great ones at that).  The definitively should not be used by drivers.
> They would be very useful buildblocks for a IOMMU cache maintainance
> API.
> 
> Of course the best outcome would be if we could find a way for the MSM
> drm driver to just use DMA API and not deal with the lower level
> abstractions.  Do you remember why the driver went for use of the IOMMU
> API?
> 

One example why drm/msm can't use DMA API is multiple page table support 
(that is landing in 5.10), which is something that definitely couldn't 
work with DMA API.

Another one is being able to choose the address for mappings, which 
AFAIK DMA API can't do (somewhat related to this: qcom hardware often 
has ranges of allowed addresses, which the dma_mask mechanism fails to 
represent, what I see is drivers using dma_mask as a "maximum address", 
and since addresses are allocated from the top it generally works)

But let us imagine drm/msm switches to using DMA API. a2xx GPUs have 
their own very basic MMU (implemented by msm_gpummu.c), that will need 
to implement dma_map_ops, which will have to call 
arch_sync_dma_for_{cpu,device}. So drm/msm still needs to call 
arch_sync_dma_for_{cpu,device} in that scenario.







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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Christoph Hellwig]

On Tue, Oct 06, 2020 at 09:19:32AM -0400, Jonathan Marek wrote:
> One example why drm/msm can't use DMA API is multiple page table support
> (that is landing in 5.10), which is something that definitely couldn't work
> with DMA API.
> 
> Another one is being able to choose the address for mappings, which AFAIK
> DMA API can't do (somewhat related to this: qcom hardware often has ranges
> of allowed addresses, which the dma_mask mechanism fails to represent, what
> I see is drivers using dma_mask as a "maximum address", and since addresses
> are allocated from the top it generally works)

That sounds like a good enough rason to use the IOMMU API.  I just
wanted to make sure this really makes sense.
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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Robin Murphy]

On 2020-10-07 07:25, Christoph Hellwig wrote:
> On Tue, Oct 06, 2020 at 09:19:32AM -0400, Jonathan Marek wrote:
>> One example why drm/msm can't use DMA API is multiple page table support
>> (that is landing in 5.10), which is something that definitely couldn't work
>> with DMA API.
>>
>> Another one is being able to choose the address for mappings, which AFAIK
>> DMA API can't do (somewhat related to this: qcom hardware often has ranges
>> of allowed addresses, which the dma_mask mechanism fails to represent, what
>> I see is drivers using dma_mask as a "maximum address", and since addresses
>> are allocated from the top it generally works)
> 
> That sounds like a good enough rason to use the IOMMU API.  I just
> wanted to make sure this really makes sense.

I still think this situation would be best handled with a variant of 
dma_ops_bypass that also guarantees to bypass SWIOTLB, and can be set 
automatically when attaching to an unmanaged IOMMU domain. That way the 
device driver can make DMA API calls in the appropriate places that do 
the right thing either way, and only needs logic to decide whether to 
use the returned DMA addresses directly or ignore them if it knows 
they're overridden by its own IOMMU mapping.

Robin.
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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Rob Clark]

On Tue, Oct 13, 2020 at 6:42 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2020-10-07 07:25, Christoph Hellwig wrote:
> > On Tue, Oct 06, 2020 at 09:19:32AM -0400, Jonathan Marek wrote:
> >> One example why drm/msm can't use DMA API is multiple page table support
> >> (that is landing in 5.10), which is something that definitely couldn't work
> >> with DMA API.
> >>
> >> Another one is being able to choose the address for mappings, which AFAIK
> >> DMA API can't do (somewhat related to this: qcom hardware often has ranges
> >> of allowed addresses, which the dma_mask mechanism fails to represent, what
> >> I see is drivers using dma_mask as a "maximum address", and since addresses
> >> are allocated from the top it generally works)
> >
> > That sounds like a good enough rason to use the IOMMU API.  I just
> > wanted to make sure this really makes sense.
>
> I still think this situation would be best handled with a variant of
> dma_ops_bypass that also guarantees to bypass SWIOTLB, and can be set
> automatically when attaching to an unmanaged IOMMU domain. That way the
> device driver can make DMA API calls in the appropriate places that do
> the right thing either way, and only needs logic to decide whether to
> use the returned DMA addresses directly or ignore them if it knows
> they're overridden by its own IOMMU mapping.
>

That would be pretty ideal from my PoV

BR,
-R
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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Christoph Hellwig]

On Tue, Oct 13, 2020 at 02:42:38PM +0100, Robin Murphy wrote:
> I still think this situation would be best handled with a variant of
> dma_ops_bypass that also guarantees to bypass SWIOTLB, and can be set
> automatically when attaching to an unmanaged IOMMU domain.

dma_ops_bypass should mostly do the right thing as-is.  swiotlb bouncing
is triggered of two things:

 1) the dma_mask.  This is under control of the driver, and obviously
    if it is too small for a legit reason we can't just proceed
 2) force_dma_unencrypted() - we'd need to do an opt-out here, either
    by a flag or by being smart and looking for an attached iommu on
    the device

> That way the
> device driver can make DMA API calls in the appropriate places that do the
> right thing either way, and only needs logic to decide whether to use the
> returned DMA addresses directly or ignore them if it knows they're
> overridden by its own IOMMU mapping.

I'd be happy to review patches for this.
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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Daniel Vetter]

On Thu, Oct 15, 2020 at 07:55:32AM +0100, Christoph Hellwig wrote:
> On Tue, Oct 13, 2020 at 02:42:38PM +0100, Robin Murphy wrote:
> > I still think this situation would be best handled with a variant of
> > dma_ops_bypass that also guarantees to bypass SWIOTLB, and can be set
> > automatically when attaching to an unmanaged IOMMU domain.
> 
> dma_ops_bypass should mostly do the right thing as-is.  swiotlb bouncing
> is triggered of two things:
> 
>  1) the dma_mask.  This is under control of the driver, and obviously
>     if it is too small for a legit reason we can't just proceed

Somewhat related, but is there a way to tell the dma-api to fail instead
of falling back to swiotlb? In many case for gpu drivers it's much better
if we fall back to dma_alloc_coherent and manage the copying ourselves
instead of abstracting this away in the dma-api. Currently that's "solved"
rather pessimistically by always allocating from dma_alloc_coherent if
swiotlb could be in the picture (at least for ttm based drivers, i915 just
falls over).
-Daniel

>  2) force_dma_unencrypted() - we'd need to do an opt-out here, either
>     by a flag or by being smart and looking for an attached iommu on
>     the device
> 
> > That way the
> > device driver can make DMA API calls in the appropriate places that do the
> > right thing either way, and only needs logic to decide whether to use the
> > returned DMA addresses directly or ignore them if it knows they're
> > overridden by its own IOMMU mapping.
> 
> I'd be happy to review patches for this.

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Christoph Hellwig]

On Thu, Oct 15, 2020 at 05:33:34PM +0200, Daniel Vetter wrote:
> On Thu, Oct 15, 2020 at 07:55:32AM +0100, Christoph Hellwig wrote:
> > On Tue, Oct 13, 2020 at 02:42:38PM +0100, Robin Murphy wrote:
> > > I still think this situation would be best handled with a variant of
> > > dma_ops_bypass that also guarantees to bypass SWIOTLB, and can be set
> > > automatically when attaching to an unmanaged IOMMU domain.
> > 
> > dma_ops_bypass should mostly do the right thing as-is.  swiotlb bouncing
> > is triggered of two things:
> > 
> >  1) the dma_mask.  This is under control of the driver, and obviously
> >     if it is too small for a legit reason we can't just proceed
> 
> Somewhat related, but is there a way to tell the dma-api to fail instead
> of falling back to swiotlb? In many case for gpu drivers it's much better
> if we fall back to dma_alloc_coherent and manage the copying ourselves
> instead of abstracting this away in the dma-api. Currently that's "solved"
> rather pessimistically by always allocating from dma_alloc_coherent if
> swiotlb could be in the picture (at least for ttm based drivers, i915 just
> falls over).

Is this for the alloc_pages plus manually map logic in various drivers?

They should switch to the new dma_alloc_pages API that I'll send to
Linus for 5.10 soon.
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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Christoph Hellwig]

On Thu, Oct 15, 2020 at 04:43:01PM +0100, Christoph Hellwig wrote:
> > Somewhat related, but is there a way to tell the dma-api to fail instead
> > of falling back to swiotlb? In many case for gpu drivers it's much better
> > if we fall back to dma_alloc_coherent and manage the copying ourselves
> > instead of abstracting this away in the dma-api. Currently that's "solved"
> > rather pessimistically by always allocating from dma_alloc_coherent if
> > swiotlb could be in the picture (at least for ttm based drivers, i915 just
> > falls over).
> 
> Is this for the alloc_pages plus manually map logic in various drivers?
> 
> They should switch to the new dma_alloc_pages API that I'll send to
> Linus for 5.10 soon.

Daniel, can you clarify this?
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Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Joerg Roedel]

On Tue, Oct 06, 2020 at 08:23:06AM +0100, Christoph Hellwig wrote:
> If people want to use the "raw" IOMMU API with not cache coherent
> devices we'll need a cache maintainance API that goes along with it.
> It could either be formally part of the IOMMU API or be separate.

The IOMMU-API does not care about the caching effects of DMA, is manages
IO address spaces for devices. I also don't know how this would be going
to be implemented, the IOMMU-API does not have the concept of handles
for mapped ranges and does not care about CPU virtual addresses (which
are needed for cache flushes) of the memory it maps into IO page-tables.

So I think a cache management API should be separate from the IOMMU-API.

Regards,

	Joerg
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