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

[Vincent Donnefort]

On Tue, Sep 08, 2026 at 11:08:26AM +0200, Thierry Reding wrote:
> 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@xxxxxxxxxx
> > > > 
> > > > ? 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

Ack. I'll keep it as is then and we will see later how to extend it, if it is
necessary.

-- 
Vincent