在 2024/4/12 0:52, T.J. Mercier 写道:
On Thu, Apr 11, 2024 at 1:21 AM Rong Qianfeng <11065417@xxxxxxxx> wrote:
在 2024/4/10 0:37, T.J. Mercier 写道:
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On Tue, Apr 9, 2024 at 12:34 AM Rong Qianfeng <11065417@xxxxxxxx> 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. 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
Thank you for the reminder.
The page pool of the system heap can meet the needs of most scenarios,
but the camera shooting scenario is quite special.
Why the camera shooting scenario needs to have a dma-buf memory pool in
the user-level？
(1) The memory demand is extremely high and time requirements are
stringent.
Preallocating for this makes sense to me, whether it is individual
buffers or one large one.
(2) The memory in the page pool(system heap) is easily reclaimed or used
by other apps.
Yeah, by design I guess. I have seen an implementation where the page
pool is proactively refilled after it has been empty for some time
which would help in scenarios where the pool is often reclaimed and
low/empty. You could add that in a vendor heap.
Yeah, a similar feature has already been implemented by vendor.
(3) High concurrent allocation and release (with deferred-free) lead to
high memory usage peaks.
Hopefully this is not every frame? I saw enough complaints about the
deferred free of pool pages that it hasn't been carried forward since
Android 13, so this should be less of a problem on recent kernels.
Additionally, after the camera exits, the shared memory pool can be
released, with minimal impact.
Why do you care about the difference here? In one case it's when the
buffer refcount goes to 0 and memory is freed immediately, and in the
other it's the next time reclaim runs the shrinker.
I'm sorry, my description wasn't clear enough. What I meant to explain
is that
the user-level dma-buf memory pool does not use reserved memory
(physically contiguous memory), and the memoryoccupation time is not too
long, resulting in a minimal impact on the system.
I don't want to add UAPI for DMA_BUF_IOCTL_SYNC_PARTIAL to Android
without it being in the upstream kernel. I don't think we can get that
without an in-kernel user of dma_buf_begin_cpu_access_partial first,
even though your use case wouldn't rely on that in-kernel usage. :\ So
if you want to add this to phones for your camera app, then I think
your best option is to add a vendor driver which implements this IOCTL
and calls the dma_buf_begin_cpu_access_partial functions which are
already exported.
Ok, thank you very much for your suggestion. I will definitely take it
into consideration.
Best,
T.J.
