lkml.org 
[lkml]   [2016]   [Sep]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] [media] vb2: map dmabuf for planes on driver queue instead of vidioc_qbuf
From
Date
Le mercredi 20 juillet 2016 à 16:20 +0300, Sakari Ailus a écrit :
> Hi Javier,
>
> On Fri, Jul 15, 2016 at 12:26:06PM -0400, Javier Martinez Canillas
> wrote:
> > The buffer planes' dma-buf are currently mapped when buffers are queued
> > from userspace but it's more appropriate to do the mapping when buffers
> > are queued in the driver since that's when the actual DMA operation are
> > going to happen.
> >
> > Suggested-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
> > Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
> >
> > ---
> >
> > Hello,
> >
> > A side effect of this change is that if the dmabuf map fails for some
> > reasons (i.e: a driver using the DMA contig memory allocator but CMA
> > not being enabled), the fail will no longer happen on VIDIOC_QBUF but
> > later (i.e: in VIDIOC_STREAMON).
> >
> > I don't know if that's an issue though but I think is worth mentioning.
>
> I have the same question has Hans --- why?
>
> I rather think we should keep the buffers mapped all the time. That'd
> require a bit of extra from the DMA-BUF framework I suppose, to support
> streaming mappings.
>
> The reason for that is performance. If you're passing the buffer between a
> couple of hardware devices, there's no need to map and unmap it every time
> the buffer is accessed by the said devices. That'd avoid an unnecessary
> cache flush as well, something that tends to be quite expensive. On a PC
> with resolutions typically used on webcams that might not really matter. But
> if you have an embedded system with a relatively modest 10 MP camera sensor,
> it's one of the first things you'll notice if you check where the CPU time
> is being spent.

That is very interesting since the initial discussion started from the
idea of adding an implicit fence wait to the map operation. This way we
could have a dma-buf fence attached without having to modify the
drivers to support it. Buffer handles could be dispatched before there
is any data in it. Though, if we keep it mapped, I believe this idea is
simply incompatible and fences should remain explicit for extra
flexibility.[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2016-09-17 09:58    [W:0.120 / U:0.984 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site