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Date: Thu, 11 Apr 2024 12:08:51 +0200
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Subject: Re: [PATCH] dma-buf: add DMA_BUF_IOCTL_SYNC_PARTIAL support
To: "T.J. Mercier" <tjmercier@google.com>
Cc: Rong Qianfeng <11065417@vivo.com>, Rong Qianfeng <rongqianfeng@vivo.com>,
 Jianqun Xu <jay.xu@rock-chips.com>, sumit.semwal@linaro.org,
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Am 10.04.24 um 17:07 schrieb T.J. Mercier:
> On Wed, Apr 10, 2024 at 7:22 AM Christian König
> <christian.koenig@amd.com> wrote:
>> Am 09.04.24 um 18:37 schrieb T.J. Mercier:
>>> On Tue, Apr 9, 2024 at 12:34 AM Rong Qianfeng <11065417@vivo.com> wrote:
>>>> 在 2024/4/8 15:58, Christian König 写道:
>>>>> Am 07.04.24 um 09:50 schrieb Rong Qianfeng:
>>>>>> [SNIP]
>>>>>>> Am 13.11.21 um 07:22 schrieb Jianqun Xu:
>>>>>>>> Add DMA_BUF_IOCTL_SYNC_PARTIAL support for user to sync dma-buf with
>>>>>>>> offset and len.
>>>>>>> You have not given an use case for this so it is a bit hard to
>>>>>>> review. And from the existing use cases I don't see why this should
>>>>>>> be necessary.
>>>>>>>
>>>>>>> Even worse from the existing backend implementation I don't even see
>>>>>>> how drivers should be able to fulfill this semantics.
>>>>>>>
>>>>>>> Please explain further,
>>>>>>> Christian.
>>>>>> Here is a practical case:
>>>>>> The user space can allocate a large chunk of dma-buf for
>>>>>> self-management, used as a shared memory pool.
>>>>>> Small dma-buf can be allocated from this shared memory pool and
>>>>>> released back to it after use, thus improving the speed of dma-buf
>>>>>> allocation and release.
>>>>>> Additionally, custom functionalities such as memory statistics and
>>>>>> boundary checking can be implemented in the user space.
>>>>>> Of course, the above-mentioned functionalities require the
>>>>>> implementation of a partial cache sync interface.
>>>>> Well that is obvious, but where is the code doing that?
>>>>>
>>>>> You can't send out code without an actual user of it. That will
>>>>> obviously be rejected.
>>>>>
>>>>> Regards,
>>>>> Christian.
>>>> In fact, we have already used the user-level dma-buf memory pool in the
>>>> camera shooting scenario on the phone.
>>>>
>>>>    From the test results, The execution time of the photo shooting
>>>> algorithm has been reduced from 3.8s to 3s.
>>>>
>>> For phones, the (out of tree) Android version of the system heap has a
>>> page pool connected to a shrinker.
>> Well, it should be obvious but I'm going to repeat it here: Submitting
>> kernel patches for our of tree code is a rather *extreme* no-go.
>>
> Sorry I think my comment led to some confusion. I wasn't suggesting
> you should take the patch; it's clearly incomplete. I was pointing out
> another option to Rong. It appears Rong is creating a single oversized
> dma-buf, and subdividing it in userspace to avoid multiple allocations
> for multiple buffers. That would lead to a need for a partial sync of
> the buffer from userspace. Instead of that, using a heap with a page
> pool gets you kind of the same thing with a lot less headache in
> userspace, and no need for partial sync. So I wanted to share that
> option, and that can go in just Android if necessary without this
> patch.

Ah! Thanks for the clarification and sorry for any noise I created.

I mean from the technical side the patch doesn't looks invalid or 
anything, but there is simply no upstream use case.

Regards,
Christian.

>
>> That in kernel code *must* have an in kernel user is a number one rule.
>>
>> When somebody violates this rule he pretty much disqualifying himself as
>> reliable source of patches since maintainers then have to assume that
>> this person tries to submit code which doesn't have a justification to
>> be upstream.
>>
>> So while this actually looks useful from the technical side as long as
>> nobody does an implementation based on an upstream driver I absolutely
>> have to reject it from the organizational side.
>>
>> Regards,
>> Christian.
>>
>>>    That allows you to skip page
>>> allocation without fully pinning the memory like you get when
>>> allocating a dma-buf that's way larger than necessary. If it's for a
>>> camera maybe you need physically contiguous memory, but it's also
>>> possible to set that up.
>>>
>>> https://android.googlesource.com/kernel/common/+/refs/heads/android14-6.1/drivers/dma-buf/heaps/system_heap.c#377
>>>
>>>
>>>> To be honest, I didn't understand your concern "...how drivers should be
>>>> able to fulfill this semantics." Can you please help explain it in more
>>>> detail?
>>>>
>>>> Thanks,
>>>>
>>>> Rong Qianfeng.
>>>>
>>>>>> Thanks
>>>>>> Rong Qianfeng.