linux-kernel - Re: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request

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Message-ID: <Pine.LNX.4.64.1203161522570.13465@axis700.grange>
Date:	Fri, 16 Mar 2012 15:28:39 +0100 (CET)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	Linus Walleij <linus.walleij@...aro.org>
cc:	Vinod Koul <vinod.koul@...el.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org,
	Jassi Brar <jassisinghbrar@...il.com>,
	Magnus Damm <magnus.damm@...il.com>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request_channel()

On Fri, 16 Mar 2012, Linus Walleij wrote:

> On Fri, Mar 16, 2012 at 12:09 PM, Guennadi Liakhovetski
> <g.liakhovetski@....de> wrote:
> 
> > And the least important question: who and when will implement the core
> > support for this?
> 
> I'm trying to call the kernel HR department to hire a consultant for me but they
> just put me on the phone queue all the time, I don't know what I might be
> doing wrong ... :-)
> 
> If the question is whether we need more people writing complicated core
> patches for the dmaengine I think the answer is "yes"?
> 
> > 1. the client issues a dma_request_channel() with _just_ a capability mask
> > and a filter and its argument as parameters - _nothing_ about channel
> > restrictions.
> >
> > 2. you propose to eliminate a filter - the core has no way to know, which
> > channel to pick up...
> 
> Nah, thinking about it...
> 
> Eliminate the external filter, make it internal. We already have the
> problem that these filter functions need to be passed around too much
> in platform data e.g. so we need to do away with it.
> 
> The filter functions seem to come from the DMA drivers
> themselves mostly. (Help me with the complete picture here...)
> For example:
> 
> amba-pl08x.c:bool pl08x_filter_id(struct dma_chan *chan, void *chan_id)
> coh901318.c:bool coh901318_filter_id(struct dma_chan *chan, void *chan_id)
> pl330.c:bool pl330_filter(struct dma_chan *chan, void *param)
> sirf-dma.c:bool sirfsoc_dma_filter_id(struct dma_chan *chan, void *chan_id)
> ste_dma40.c:bool stedma40_filter(struct dma_chan *chan, void *data)
> 
> So delete the typedef for dma_filter_fn remove these filters from
> external header files.
> 
> And stop that thing from being passed around and into
> struct dma_device so the dmaengine core can still filter or process
> channels, but nothing on the outside need to know about it. That way
> we can centralize it to drivers/dma and not spread it out throughout
> the kernel.
> 
> > 3. the wrapper, proposed by Russell, now calls dmaengine_slave_config(),
> > which fails, because that's a wrong channel (hope I get this right this
> > time - configuration has nothing to do with selection:-))
> 
> Oh I was not thinking of relying on config to sort out channels.
> 
> I was thinking of internalizing the dma_filter_fn and make it an
> (optional, maybe?) part of dmaengine.

Yessss!!! Let's do that! :-D Now, you're proposing exactly the same, as 
what I was proposing! :-) Now you just have to remove the filter function 
parameter from dma_request_channel() - it is anyway the same for all and 
implemented in the dmaengine core - and you get

dma_request_channel(mask, slave_desc)

which is exactly what I was proposing! :-) Ok, I didn't remove the filter 
function, instead I added one more parameter, but in essence - it is the 
same! But since you yourself say, that this isn't easy - to remove the 
filter function from all drivers at once, maybe my variant - add a 
parameter and transition gradually - is easier! ;-)

Thanks
Guennadi

> > 4. that's it, if you start again - the dmaengine core will enumerate the
> > same channels again and propose the same unsuitable channel to you -
> > there's no way to continue to the next channel / device.
> >
> > What am I missing? How is the mapping going to be used, if you eliminate
> > the filter function?
> 
> As above, I guess factoring away the filter functions would be
> the first real hard problem.
> 
> Thanks,
> Linus Walleij
> 

---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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