linux-kernel - Re: [PATCH 2.6.20 1/1] fbdev,mm: hecuba/E-Ink fbdev driver

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Date:	Tue, 20 Feb 2007 13:38:48 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Jaya Kumar <jayakumar.lkml@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-fbdev-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jsimmons@...radead.org, Geert.Uytterhoeven@...ycom.com
Subject: Re: [PATCH 2.6.20 1/1] fbdev,mm: hecuba/E-Ink fbdev driver

On Mon, Feb 19, 2007 at 11:13:04PM -0500, Jaya Kumar wrote:
> On 2/18/07, Paul Mundt <lethal@...ux-sh.org> wrote:
> >Given that, this would have to be something that's dealt with at the
> >subsystem level rather than in individual drivers, hence the desire to
> >see something like this more generically visible.
> >
> 
> Hi Peter, Paul, fbdev folk,
> 
> Ok. Here's what I'm thinking for abstracting this:
> 
> fbdev drivers would setup fb_mmap with their own_mmap as usual. In
> own_mmap, they would do what they normally do and setup a vm_ops. They
> are free to have their own nopage handler but would set the
> page_mkwrite handler to be fbdev_deferred_io_mkwrite().

The vast majority of drivers do not implement ->fb_mmap(), and with
proper abstraction, this should be something that's possible as a direct
alternative to drivers/video/fbmem.c:fb_mmap() for the people that want
it. Of course it's just as easy to do something like the sbuslib.c route
and then have drivers set their ->fb_mmap() from that too.

> fbdev_deferred_io_mkwrite would build up the list of touched pages and
> pass it to a delayed workqueue which would then mkclean on each page
> and then pass a copy of that page list down to a driver's callback
> function. The fbdev driver's callback function can then do the actual
> IO to the framebuffer or coalesce DMA based on the provided page list.
> 
That works for me, though I'd prefer for struct page_list to be done with
a scatterlist, then it's trivial to setup from the workqueue context
without having to shuffle things around.
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