2013/5/15 Rob Clark robdclark@gmail.com
On Wed, May 15, 2013 at 1:19 AM, Inki Dae inki.dae@samsung.com wrote:
-----Original Message----- From: Rob Clark [mailto:robdclark@gmail.com] Sent: Tuesday, May 14, 2013 10:39 PM To: Inki Dae Cc: linux-fbdev; DRI mailing list; Kyungmin Park; myungjoo.ham; YoungJun Cho; linux-arm-kernel@lists.infradead.org; linux-media@vger.kernel.org Subject: Re: Introduce a new helper framework for buffer synchronization
On Mon, May 13, 2013 at 10:52 PM, Inki Dae inki.dae@samsung.com
wrote:
well, for cache management, I think it is a better idea.. I didn't really catch that this was the motivation from the initial patch, but maybe I read it too quickly. But cache can be decoupled from synchronization, because CPU access is not asynchronous. For userspace/CPU access to buffer, you should:
- wait for buffer
- prepare-access
- ... do whatever cpu access to buffer ...
- finish-access
- submit buffer for new dma-operation
For data flow from CPU to DMA device,
- wait for buffer
- prepare-access (dma_buf_begin_cpu_access)
- cpu access to buffer
For data flow from DMA device to CPU
- wait for buffer
Right, but CPU access isn't asynchronous (from the point of view of the CPU), so there isn't really any wait step at this point. And if you do want the CPU to be able to signal a fence from userspace for some reason, you probably what something file/fd based so the refcnting/cleanup when process dies doesn't leave some pending DMA action wedged. But I don't really see the point of that complexity when the CPU access isn't asynchronous in the first place.
There was my missing comments, please see the below sequence.
For data flow from CPU to DMA device and then from DMA device to CPU,
- wait for buffer <- at user side - ioctl(fd, DMA_BUF_GET_FENCE, ...) - including prepare-access (dma_buf_begin_cpu_access)
- cpu access to buffer
- wait for buffer <- at device driver - but CPU is already accessing the buffer so blocked.
- signal <- at user side - ioctl(fd, DMA_BUF_PUT_FENCE, ...)
- the thread, blocked at 3), is waked up by 4). - and then finish-access (dma_buf_end_cpu_access)
right, I understand you can have background threads, etc, in userspace. But there are already plenty of synchronization primitives that can be used for cpu->cpu synchronization, either within the same process or between multiple processes. For cpu access, even if it is handled by background threads/processes, I think it is better to use the traditional pthreads or unix synchronization primitives. They have existed forever, they are well tested, and they work.
So while it seems nice and orthogonal/clean to couple cache and synchronization and handle dma->cpu and cpu->cpu and cpu->dma in the same generic way, but I think in practice we have to make things more complex than they otherwise need to be to do this. Otherwise I think we'll be having problems with badly behaved or crashing userspace.
Right, this is very important. I think it's not esay to solve this issue. Aand at least for ARM based embedded system, such feature is useful to us; coupling cache operation and buffer synchronization. I'd like to collect other opinions and advices to solve this issue.
Thanks, Inki Dae
BR, -R
- dma access to buffer
- wait for buffer <- at user side - ioctl(fd, DMA_BUF_GET_FENCE, ...) - but DMA is already accessing the buffer so blocked.
- signal <- at device driver
- the thread, blocked at 7), is waked up by 8) - and then prepare-access (dma_buf_begin_cpu_access)
10 cpu access to buffer
Basically, 'wait for buffer' includes buffer synchronization, committing processing, and cache operation. The buffer synchronization means that a current thread should wait for other threads accessing a shared buffer
until
the completion of their access. And the committing processing means that
a
current thread possesses the shared buffer so any trying to access the shared buffer by another thread makes the thread to be blocked. However,
as
I already mentioned before, it seems that these user interfaces are so
ugly
yet. So we need better way.
Give me more comments if there is my missing point :)
Thanks, Inki Dae
BR, -R
finish-access (dma_buf_end _cpu_access)
dma access to buffer
and 2) are coupled with one function: we have implemented
fence_helper_commit_reserve() for it.
Cache control(cache clean or cache invalidate) is performed properly checking previous access type and current access type. And the below is actual codes for it,
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