On Sat, May 18, 2013 at 03:47:43PM +0900, Inki Dae wrote:
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.
Maybe we have a bit a misunderstanding here. The kernel really should take care of sync and cache coherency, and I agree that with the current dma_buf code (and the proposed fences) that part is still a bit hazy. But the kernel should not allow userspace to block access to a buffer until userspace is done. It should only sync with any oustanding fences and flush buffers before that userspace access happens.
Then the kernel would again flush caches on the next dma access (which hopefully is only started once userspace completed access). Atm this isn't possible in an efficient way since the dma_buf api only exposes map/unmap attachment and not a function to just sync an existing mapping like dma_sync_single_for_device. I guess we should add a dma_buf_sync_attachment interface for that - we already have range-based cpu sync support with the begin/end_cpu_access interfaces.
Yours, Daniel