Re: [PATCH] dma-buf: heaps: Introduce dma_heap_add_cma() for non-default CMA heap

[Kunihiko Hayashi]

On 2020/08/01 4:38, John Stultz wrote:
> On Fri, Jul 31, 2020 at 2:32 AM Kunihiko Hayashi
> <hayashi.kunihiko@xxxxxxxxxxxxx> wrote:
> > On 2020/07/29 4:17, John Stultz wrote:
> > > Do you have a upstream driver that you plan to make use this new call?
> >
> > Unfortunately I don't have an upstream driver using this call.
> >
> > This call is called from dma-buf heaps "importer" or "customer",
> > and I only made an example (do nothing) importer driver
> > to test the call.
> >
> > > We want to have in-tree users of code added.
> >
> > I think this is a generic way to use non-default CMA heaps, however,
> > we need in-tree "importer" drivers to want to use non-default CMA heaps.
> > I don't find it from now.
>
> Yea, I and again, I do agree this is functionality that will be
> needed. But we'll need to wait for a user (camera driver, etc which
> would utilize the reserved cma region) before we can merge it
> upstream. :(  Do let me know if you have an out of tree driver that
> would make use of it, and we can see what can be done to help upstream
> things.

Sorry for late.
Before I prepare or find a user driver as "importer",
I think something is different in this patch.

This patch makes it possible to treat non-default CMA connected to
"importer" device with memory-region as dma-buf heaps.

However, the allocated memory from this dma-buf heaps can be used
for "any" devices, and the "importer" can treat memories from other
dma-buf heaps.

So, the "importer" and the non-default CMA aren't directly related,
and I think an "exporter" for the non-default CMA should be enabled.

In paticular, the kernel initializer (as an "exporter") calls
dma_heap_add_cma() for all CMAs defined in Devicetree, and
the device files associated with each CMA appear under "/dev/dma_heap/".
For example:

   /dev/dma_heap/linux,cma@10000000
   /dev/dma_heap/linux,cma@11000000
   /dev/dma_heap/linux,cma@12000000
   ...

All of these device files can be fairly allocated to any "importer" device.

Actually I think that the kernel should executes dma_heap_add_cma()
for ALL defined reserved-memory nodes.

If this idea hasn't been discussed yet and this is reasonable,
I'll prepare RFC patches.

Thank you,

---
Best Regards
Kunihiko Hayashi