Re: [PATCH v6 00/12] dma-buf: heaps: Add support for Tegra VPR

[Thierry Reding <thierry.reding@kernel.org>]

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On Tue, Sep 08, 2026 at 09:57:57AM +0100, Vincent Donnefort wrote:
> On Tue, Sep 08, 2026 at 10:39:17AM +0200, Thierry Reding wrote:
> > On Fri, Sep 04, 2026 at 12:41:05PM +0100, Will Deacon wrote:
> > > Hi Thierry,
> > > 
> > > On Fri, Sep 04, 2026 at 12:44:51PM +0200, Thierry Reding wrote:
> > > > This series adds support for the video protection region (VPR) used on
> > > > Tegra SoC devices. It's a special region of memory that is protected
> > > > from accesses by the CPU and used to store DRM protected content (both
> > > > decrypted stream data as well as decoded video frames).
> > > > 
> > > > Patches 1 through 3 add DT binding documentation for the VPR and add the
> > > > VPR to the list of memory-region items for display, host1x and NVDEC.
> > > > 
> > > > The set_direct_map_*_noflush() functions that will be used later in this
> > > > series are exported in patch 4 so that the drivers that use them can be
> > > > built as a module.
> > > > 
> > > > Patch 5 adds bitmap_allocate(), which is like bitmap_allocate_region()
> > > > but works on sizes that are not a power of two.
> > > > 
> > > > The of_node_to_nid() function is exported in patch 6 because it is used
> > > > in a later patch adding a driver that can be built as a module.
> > > > 
> > > > Patch 7 introduces new APIs needed by the Tegra VPR implementation that
> > > > allow memory to be allocated at a fixed offset within a CMA area. Tegra
> > > > VPR needs this in order to implement its own allocator on top of CMA to
> > > > meet the strict hardware requirements. This replaces the dynamic CMA
> > > > area creation patch from earlier versions.
> > > 
> > > Did you get a chance to see how this could work with Vincent's series:
> > > 
> > > https://lore.kernel.org/r/20260902104712.2399797-1-vdonnefort@google.com
> > > 
> > > ? I think that should remove your reliance on can_set_direct_map() and
> > > mean that you can retain block mappings for most of the linear mapping.
> > 
> > I'm not sure if it would help all that much. Yes, if we mark the VPR
> > region as LLMAP (or PTE_MAP, whichever it ends up being), it should make
> > the checks for can_set_direct_map() redundant. However, from what I can
> > tell, Vincent's series still forces page-granularity on these regions,
> > so it won't retain block mappings at all for them.
> > 
> > The block mappings can be retained for the non-VPR memory, so that's
> > nice. It also reduces the amount of external prerequisites, but I had
> > kind of hoped that we could go one step further and keep block mappings
> > even for the VPR memory if the region happened to be a multiple of the
> > block size.
> > 
> > The recent addition of page count to the set_direct_map_*() functions
> > helps reduce the amount of checks that need to be run, so maybe there's
> > not too much to be gained from removing whole block mappings at once
> > from the linear map.
> > 
> > Thierry
> 
> I should be able to add PMD_SIZE mapping support to the series. That was
> actually my original idea as we can easily force the CMA allocation granule to
> be PMD_SIZE too.
> 
> I didn't implement it as I thought there were not much interest in the end (and
> also as contiguous.c is always using PAGE_SIZE granularity).
> 
> But now as I have implemented a specific pool (and do not use contiguous.c as
> originally planned), if you believe it is important for the VPR driver, let me
> see if I can extend the support in a V2.

I don't think it needs to be part of a v2 and can be a follow-up. It
should be transparent from an API point of view and merely be an
optimisation for that specific case.

Eventually it'd be nice to have, though it might also be worth checking
what the actually gains are.

Thierry

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