[Linaro-mm-sig] [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

Semwal, Sumit
2011-12-19T06:16:40Z

Hi Arnd, Daniel,

On Mon, Dec 12, 2011 at 10:18 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Saturday 10 December 2011, Daniel Vetter wrote:
>> If userspace (through some driver calls)
>> tries to do stupid things, it'll just get garbage. See
>> Message-ID: <CAKMK7uHeXYn-v_8cmpLNWsFY14KtmuRZy8YRKR5Xst2-2WdFSQ@mail.gmail.com>
>> for my reasons why it think this is the right way to go forward. So in
>> essence I'm really interested in the reasons why you want the kernel
>> to enforce this (or I'm completely missing what's the contentious
>> issue here).
>
> This has nothing to do with user space mappings. Whatever user space does,
> you get garbage if you don't invalidate cache lines that were introduced
> through speculative prefetching before you access cache lines that were
> DMA'd from a device.
I didn't see a consensus on whether dma_buf should enforce some form
of serialization within the API - so atleast for v1 of dma-buf, I
propose to 'not' impose a restriction, and we can tackle it (add new
ops or enforce as design?) whenever we see the first need of it - will
that be ok? [I am bending towards the thought that it is a problem to
solve at a bigger platform than dma_buf.]
>
> ? ? ? ?Arnd

Best regards,
~Sumit.
>
> --