linux-kernel - Re: [PATCH 07/13] dmaengine: introduce dma_request

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Message-ID: <Pine.LNX.4.64.0812021820560.5663@axis700.grange>
Date:	Tue, 2 Dec 2008 18:27:26 +0100 (CET)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	Dan Williams <dan.j.williams@...el.com>
cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	maciej.sosnowski@...el.com, hskinnemoen@...el.com,
	nicolas.ferre@...el.com
Subject: Re: [PATCH 07/13] dmaengine: introduce dma_request_channel and
 private channels

On Tue, 2 Dec 2008, Dan Williams wrote:

> On Tue, Dec 2, 2008 at 8:52 AM, Guennadi Liakhovetski
> <g.liakhovetski@....de> wrote:
> > Hi Dan,
> >
> > I think, there is a problem with your dma_request_channel() /
> > private_candidate() implementation: your current version only tries one
> > channel from a dma device list, which matched capabilities. If this
> > channel is not accepted by the client, you do not try other channels from
> > this device and just go to the next one...
> >
> 
> Which dma driver are you using?

This is the idmac dmaengine driver I submitted a few weeks ago, that I am 
porting to your modified dmaengine framework. Initial version:

http://marc.info/?l=linux-arm-kernel&m=122607472721145&w=2

BTW - it does look nicer and more simple now, so, in general, I like the 
change.

> The dmaengine code assumes that all
> channels on a device are equal.  It sounds like there are differences
> between peer-channels on the device in this case.  If the driver
> registers a device per channel that should give the flexibility you
> want.

Ooh... Do you really think registering 32 dma-devices is a better solution 
than allowing non-equal dma-channels? As I explained in the commit 
comment, this is a specialised Image Processing DMA Controller, and each 
its channel has a fixed role. So, each client has to get a specific 
channel.

> > Another problem I encountered with my framebuffer is the initialisation
> > order. You initialise dmaengine per subsys_initcall(), whereas the only
> > way to guarantee the order:
> >
> > dmaengine
> > dma-device driver
> > framebuffer
> 
> hmm... can the framebuffer be moved to late_initcall?

I assumed, that one wants to register the framebuffer as early as 
possible...

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
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