Re: [PATCH v4 07/10] dma-buf: heaps: Add support for Tegra VPR

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, Aug 21, 2026 at 09:44:49PM +0100, Vincent Donnefort wrote:
> [...]
> 
> > > > Interesting. I had been thinking more along the lines of adding this
> > > > directly into memblock using a new entry in enum memblock_flags. None of
> > > > the existing ones seem to do what we need here, though some are closely
> > > > related. MEMBLOCK_SECURE or MEMBLOCK_PROTECTED are quite specific and
> > > > don't necessarily mean that we need the mapping to be page-granular, so
> > > > MEMBLOCK_PAGE_GRANULAR is perhaps clearer.
> > > 
> > > I thought it would be interesting to not have to modify memblock since it is
> > > used for all architectures, while the DT is at least slightly less widespread.
> > 
> > This isn't something that is DT specific. On the Tegra side, I suspect
> > we might need this on ACPI platforms eventually, too. And it's not an
> > ARM specific feature either. Other platforms could use shared carveouts,
> > too.
> 
> Agreed, and actually I have tried the memblock version and it looks quite neat!
> 
> I will share what I have after the merge-window.
> 
> > 
> > > Also, cma_init_reserved_mem() could use an order 9. In that case PTE-level isn't
> > > necessary and we could just use PMD-level. Extending that support would be more
> > > cumbersome with a memblock flag than with a callback.
> > > 
> > > rmem_cma_setup() hardcoding an order 0, perhaps this isn't really a problem at
> > > the moment?
> > 
> > How so? memblock is really just used as a way of backing the CMA. How
> > CMA subdivides it doesn't really matter, right?
> > 
> > Or are you suggesting that if we use an entire PMD as granularity, we
> > could equally well remove the entire PMD from the linear mapping instead
> > of doing it page-by-page? I think that'd work really well for at least
> > VPR, since it is 1 MiB aligned anyway. Using a granularity of 2 MiB is
> > easily doable.
> 
> Yes, that is what I meant. We could easily force a PMD-level mapping instead of
> PTE-level one. But then that means declaring another memblock flag. 
> 
> > 
> > For anything that doesn't require a single contiguous area to be
> > protected this might be a bit more challenging since it potentially
> > wastes a lot of memory. On the other hand, a lot of this is heavily
> > custom code anyway, so the entire stack could be modified to make
> > efficient use of this (i.e. userspace could allocate a larger chunk
> > for a pool of buffers, etc.).
> > 
> > > But yeah, the alternative is to create a "memblock_mark_forcepte" (it seems
> > > memblock_setclr_flag does split memblocks) and let of_reserved_mem call that
> > > function. Finally the arm64 mmu code can simply check for the flag before
> > > calling __map_memblock. Perhaps it isn't that bad in the end?
> > 
> > It sounds like the right level of abstraction to me. But I'm not too
> > familiar with this code, so it'd be good to hear from the MM and/or ARM
> > maintainers what they think about this.
> > 
> > [...]
> > > I had in mind to extend "shared-dma-pool" to handle
> > > set_direct_map_invalid_noflush()/set_direct_map_default_noflush() based on an
> > > option. But perhaps it is better to create another separate driver. And VPR
> > > needing a specific dma-heap driver anyway, it could call the direct-map
> > > functions there too without relying on CMA to do anything?
> > 
> > I think it'd be nice to have separate APIs for this case where we know
> > the memory region is already page-granular (or, I suppose, PMD granular)
> > and removing from (or adding back to) the linear mapping is safe. That
> > way we could avoid the checks for can_set_direct_map() for each page.
> > 
> > It'd also be nice to have a version that can update the protection bits
> > for a range of pages (would map directly to update_range_prot()) instead
> > of having to manually iterate over each page in a range.
> >
> 
> How about?
> 
>   /* True if can_set_direct_map() or [start, end) is mapped at PTE-level */
>   can_set_direct_map_range(phys_addr_t start, phys_addr_t end); 
> 
>   /* Must check can_set_direct_map() or can_set_direct_map_range() first */
>   __set_direct_map_invalid_noflush(phys_addr_t start, phys_addr_t end)
>   __set_direct_map_default_noflush(phys_addr_t start, phys_addr_t end)
> 
> > A good middle-ground might be to have helpers that do the grunt work and
> > they can then be called from VPR and the shared-dma-pool drivers to have
> > pages removed from the linear mapping. VPR and similar can then perform
> > the hardware protection bits on top of that.
> 
> I believe between the reserved-memory attribute to force the last-level mappings
> and the direct map functions above, there's enough for the VPR driver?
> 
> -- 
> Vincent
> 
> > 
> > Thierry

I have pushed a first version here [1]. I am waiting for the merge window to end
before posting anything on the list, but let me know if you have any comment.

[1] https://android-kvm.googlesource.com/linux/+/refs/heads/vdonnefort/ffa-lend-pool

-- 
Vincent