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SubjectRe: [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism
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On Mon, Dec 5, 2011 at 4:09 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Monday 05 December 2011 14:46:47 Rob Clark wrote:
>> I sort of preferred having the DMABUF shim because that lets you pass
>> a buffer around userspace without the receiving code knowing about a
>> device specific API.  But the problem I eventually came around to: if
>> your GL stack (or some other userspace component) is batching up
>> commands before submission to kernel, the buffers you need to wait for
>> completion might not even be submitted yet.  So from kernel
>> perspective they are "ready" for cpu access.  Even though in fact they
>> are not in a consistent state from rendering perspective.  I don't
>> really know a sane way to deal with that.  Maybe the approach instead
>> should be a userspace level API (in libkms/libdrm?) to provide
>> abstraction for userspace access to buffers rather than dealing with
>> this at the kernel level.
>
> It would be nice if user space had no way to block out kernel drivers,
> otherwise we have to be very careful to ensure that each map() operation
> can be interrupted by a signal as the last resort to avoid deadlocks.

map_dma_buf should be documented to be allowed to return -EINTR..
otherwise, yeah, that would be problematic.

>        Arnd
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