[PATCH v4] dma-buf: Add ioctls to allow userspace to flush

Thomas Hellstrom thellstrom at vmware.com
Wed Aug 26 05:28:30 PDT 2015

On 08/26/2015 02:10 PM, Daniel Vetter wrote:
> On Wed, Aug 26, 2015 at 08:49:00AM +0200, Thomas Hellstrom wrote:
>> Hi, Tiago.
>> On 08/26/2015 02:02 AM, Tiago Vignatti wrote:
>>> From: Daniel Vetter <daniel.vetter at ffwll.ch>
>>> The userspace might need some sort of cache coherency management e.g. when CPU
>>> and GPU domains are being accessed through dma-buf at the same time. To
>>> circumvent this problem there are begin/end coherency markers, that forward
>>> directly to existing dma-buf device drivers vfunc hooks. Userspace can make use
>>> of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence would be
>>> used like following:
>>>   - mmap dma-buf fd
>>>   - for each drawing/upload cycle in CPU
>>>     1. SYNC_START ioctl
>>>     2. read/write to mmap area or a 2d sub-region of it
>>>     3. SYNC_END ioctl.
>>>   - munamp once you don't need the buffer any more
>>> v2 (Tiago): Fix header file type names (u64 -> __u64)
>>> v3 (Tiago): Add documentation. Use enum dma_buf_sync_flags to the begin/end
>>> dma-buf functions. Check for overflows in start/length.
>>> v4 (Tiago): use 2d regions for sync.
>> Daniel V had issues with the sync argument proposed by Daniel S. I'm
>> fine with that argument or an argument containing only a single sync
>> rect. I'm not sure whether Daniel V will find it easier to accept only a
>> single sync rect...
> I'm kinda against all the 2d rect sync proposals ;-) At least for the
> current stuff it's all about linear subranges afaik, and even there we
> don't bother with flushing them precisely right now.
> My expectation would be that if you _really_ want to etch out that last
> bit of performance with a list of 2d sync ranges then probably you want to
> do the cpu cache flushing in userspace anyway, with 100% machine-specific
> trickery.


I might be misunderstanding things, but isn't this about finally
accepting a dma-buf mmap() generic interface for people who want to use
it for zero-copy applications (like people have been trying to do for
years but never bothered to specify an interface that actually performed
on incoherent hardware)?

If it's only about exposing the kernel 1D sync interface to user-space
for correctness, then why isn't that done transparently to the user?


> -Daniel

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