linux-kernel - Re: AT32 ASoC Driver Patches on alsa-devel

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Date:	Fri, 6 Jun 2008 09:32:15 -0500
From:	Geoffrey Wossum <geoffrey@...er.net>
To:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
Cc:	kernel@...32linux.org, linux-kernel@...r.kernel.org
Subject: Re: AT32 ASoC Driver Patches on alsa-devel

On Friday 06 June 2008 04:29:15 am Haavard Skinnemoen wrote:
> Geoffrey Wossum <geoffrey@...er.net> wrote:
> > On Thursday 05 June 2008 11:24:09 am Haavard Skinnemoen wrote:
> > > Geoffrey Wossum <geoffrey@...er.net> wrote:
> > > > On Thursday 05 June 2008 09:22:06 am Haavard Skinnemoen wrote:
> > > > > Geoffrey Wossum <geoffrey@...er.net> wrote:

> Ok. Since the SSC module is a multi-purpose module, the SSC driver was
> designed to be as thin and flexible as possible. So we just added some
> platform_driver glue and a request/free interface and left everything
> else up to the higher-level driver.
>
> I suppose we could add a few more helper functions. Suggestions are
> welcome.

It's hard to try to add all the possible permutations of functionality.  
Probably just getting register bit field constants defined would help out 
tremendously. 

> >  One issue is the SSC
> > interface, which you have said could be used on the AT91.  After that,
> > the main differences are that the DMA buffer has to be allocated
> > differently, and the mmap() implementation is different.
>
> Oh. That's a bit surprising to me...can you elaborate on the
> differences?

Well, I used a coherent DMA buffer on the AVR32.  I was trying to rule out 
caching effects for the issues I was having at first.  Eventually it should 
be switched back to a writecombined DMA buffer.  I assume this will require 
adding some dma_sync_*() calls, which strangely seem to be missing from  
at91-pcm.c.

The mmap() would still look different, though.  The AT91 uses  
dma_mmap_writecombine(), which only exists on the ARM.  Glancing at that 
routine, it seems to jump through a lot of hoops.  I've never worked on an 
ARM with an MMU, so I don't really know why it's necessary.  But the AVR32 
just requires a call to remap_pfn_range().

---
Geoffrey

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