Re: [PATCH v6 7/9] dma-buf: system_heap: Enforce shared-granule alignment for cc-shared buffers

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Sep 21, 2026 at 11:07:17AM +0200, Christian König wrote:
> On 9/18/26 18:53, Jason Gunthorpe wrote:
> > On Fri, Sep 18, 2026 at 05:39:55PM +0200, Christian König wrote:
> > 
> >>> Arch code can figure out how to do it. If some ARM configs only give
> >>> order 4 folios or whatever then dmabuf heap doesn't care.
> >>
> >> The fundamental problem is that DMA allocations are highly
> >> architecture and device specific while Linux memory allocation APIs
> >> are generic.
> > 
> > This isn't a dma allocation, this is a memory allocation.
> 
> No, I mean this is a DMA-buf heaps allocation. It is a DMA
> allocation, we just don't know for which device.

So? How is it any different from the existing alloc pages?
 
> So ideally we should use resources which work for most devices in
> the system.

Which this does.

> If a device has special allocation requirements (CMA,
> restricted addressing etc...) we need a specialized DMA-buf heaps
> for it.

Those things don't really intersect with CC, but if they did their are
already heap names to request those, someone can add some shared
restricted CMA option if they need someday ?

> That userspace provides this cc_shared flag is a NO-GO to begin
> with.

What do you mean? We discussed this with the heap maintainers and we
all agreed this was a kind of heap just like any of the other kinds of
heaps that userspace can request.

It is *exactly* the "special allocation requirements" you are talking
about above.

Jason