linux-kernel - Re: [PATCH] dma-buf: heaps: Introduce dma_heap_add

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Date:   Fri, 21 Aug 2020 18:14:01 +0900
From:   Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     Sumit Semwal <sumit.semwal@...aro.org>,
        "Andrew F . Davis" <afd@...com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Laura Abbott <labbott@...hat.com>,
        Brian Starkey <Brian.Starkey@....com>,
        Christian Koenig <christian.koenig@....com>,
        linux-media@...r.kernel.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma-buf: heaps: Introduce dma_heap_add_cma() for
 non-default CMA heap

On 2020/08/01 4:38, John Stultz wrote:
> On Fri, Jul 31, 2020 at 2:32 AM Kunihiko Hayashi
> <hayashi.kunihiko@...ionext.com> 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@...00000
    /dev/dma_heap/linux,cma@...00000
    /dev/dma_heap/linux,cma@...00000
    ...

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