Am 10.04.24 um 17:07 schrieb T.J. Mercier:
On Wed, Apr 10, 2024 at 7:22 AM Christian König
<christian.koenig@xxxxxxx> wrote:
Am 09.04.24 um 18:37 schrieb T.J. Mercier:
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.
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.
