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From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: =?ISO-8859-1?Q?R=E9mi?= Denis-Courmont <remi@remlab.net>
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Subject: Re: [PATCHv2 3/9] v4l: add buffer exporting via dmabuf
Date: Tue, 31 Jul 2012 18:28:12 +0200
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In-Reply-To: <201207311639.02693.remi@remlab.net>
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Hi Rémi,

On Tuesday 31 July 2012 16:39:00 Rémi Denis-Courmont wrote:
> Le mardi 31 juillet 2012 14:56:14 Laurent Pinchart, vous avez écrit :
> > > For that matter, wouldn't it be useful to support exporting a userptr
> > > buffer at some point in the future?
> > 
> > Shouldn't USERPTR usage be discouraged once we get dma-buf support ?
> 
> USERPTR, where available, is currently the only way to perform zero-copy
> from kernel to userspace. READWRITE does not support zero-copy at all. MMAP
> only supports zero-copy if userspace knows a boundary on the number of
> concurrent buffers *and* the device can deal with that number of buffers;
> in general, MMAP requires memory copying.

Could you please share your use case(s) with us ?

> I am not sure DMABUF even supports transmitting data efficiently to
> userspace. In my understanding, it's meant for transmitting data between
> DSP's bypassing userspace entirely, in other words the exact opposite of
> what USERBUF does.

-- 
Regards,

Laurent Pinchart