linux-kernel - Re: How to efficiently handle DMA and cache on ARMv7 ? (was " Is get_user_pages() enough to prevent pages from being swapped out ?")

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Message-Id: <200908071213.48523.laurent.pinchart@ideasonboard.com>
Date:	Fri, 7 Aug 2009 12:13:48 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Matthieu CASTET <matthieu.castet@...rot.com>
Cc:	David Xiao <dxiao@...adcom.com>, Ben Dooks <ben-linux@...ff.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Robin Holt <holt@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"v4l2_linux" <linux-media@...r.kernel.org>,
	"linux-arm-kernel@...ts.arm.linux.org.uk" 
	<linux-arm-kernel@...ts.arm.linux.org.uk>
Subject: Re: How to efficiently handle DMA and cache on ARMv7 ? (was " 
 Is	get_user_pages() enough to prevent pages from being swapped out ?")

On Friday 07 August 2009 10:12:23 Matthieu CASTET wrote:
> Laurent Pinchart a écrit :
> > On Thursday 06 August 2009 20:46:14 David Xiao wrote:
> >
> > Think about the simple following use case. An application wants to
> > display video it acquires from the device to the screen using Xv. The
> > video buffer is allocated by Xv. Using the v4l2 user pointer streaming
> > method, the device can DMA directly to the Xv buffer. Using
> > driver-allocated buffers, a memcpy() is required between the v4l2 buffer
> > and the Xv buffer.
>
> v4l2 got an API (overlay IRRC) that allow drivers to write directly in
> framebuffer memory.

That's right, but I was mostly using this as an example.

> BTW Xv buffer is not always in video memory and the X driver can do a
> memcpy.

Still, one less memcpy is better :-)

Regards,

Laurent Pinchart

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