Re: [RFC/PATCH 0/6] DSPBRIDGE: fix mem+cache API issues

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On Fri, May 21, 2010 at 9:14 AM, Felipe Contreras
<felipe.contreras@xxxxxxxxx> wrote:
> On Fri, May 21, 2010 at 12:22 AM, Ohad Ben-Cohen <ohad@xxxxxxxxxx> wrote:
>> On Wed, May 19, 2010 at 7:50 PM, Felipe Contreras
>> <felipe.contreras@xxxxxxxxx> wrote:
>>> I don't know what VM_IO means, but essentially we don't want to go
>>> find each and page when we know the memory is contiguous. Although the
>>> fact that the memory is writecombine would help us avoid unnecessary
>>> flushes as well :)
>>
>> Yes, I guess you can remove those calls from the application :)
>
> Can't. From GStreamer point of view the decoder is independent of the
> renderer, so we have no way to know that.

Can you please elaborate on this a little ?

> Perhaps if the memory map
> operation returned some information we can avoid those calls.

I don't think it should; relying on the map call to tell us what type
of buffer we just gave it smells like a bad interface. After all, it
is the application who originally called mmap() to get access to that
buffer, so it already knows if it's a framebuffer of not.

Anyway it's not that an important optimization to do: the ioctl will
just fail. The negative side is that the application will have no way
to know if this is really a failure (did I just give a wrong buffer
address to dspbridge ?) or not (oh it's the framebuffer, there's no
mapping for that).

Performance-wise, it's better than the previous implementation which
would have executed redundant cache operation on the buffer.

Thanks,
Ohad.
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