linux-kernel - Re: [PATCH 07/11] spi/pxa2xx: add support for DMA engine

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Date:	Thu, 17 Jan 2013 12:39:13 +0200
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Vinod Koul <vinod.koul@...el.com>, Dan Williams <djbw@...com>,
	linux-kernel@...r.kernel.org, grant.likely@...retlab.ca,
	eric.y.miao@...il.com, linux@....linux.org.uk,
	haojian.zhuang@...il.com, broonie@...nsource.wolfsonmicro.com,
	chao.bi@...el.com, "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 07/11] spi/pxa2xx: add support for DMA engine

On Thu, Jan 17, 2013 at 10:48:15AM +0100, Linus Walleij wrote:
> On Mon, Jan 7, 2013 at 11:44 AM, Mika Westerberg
> <mika.westerberg@...ux.intel.com> wrote:
> 
> > In order to use DMA with this driver on non-PXA platforms we implement support
> > for the generic DMA engine API. This allows to use different DMA engines with
> > little or no modification to the driver.
> >
> > Request lines and channel numbers can be passed to the driver from the
> > platform specific data.
> >
> > Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > ---
> >  drivers/spi/spi-pxa2xx.c       |  351 +++++++++++++++++++++++++++++++++++++++-
> >  include/linux/spi/pxa2xx_spi.h |    6 +
> 
> So if the other DMA implementation could go into spi-pxa2xx-pxadma.c
> this could be spi-pxa2xx-dma.c and they could use the common header
> file spi-pxa2xx-dma.h and then Kconfig select which one to compile.
> 
> Just a suggestion to get the driver more readable.

Yes, it makes perfect sense. I'll do that in the next version.

> > +       /*
> > +        * Some DMA controllers have problems transferring buffers that are
> > +        * not multiple of 4 bytes. So we truncate the transfer so that it
> > +        * is suitable for such controllers, and handle the trailing bytes
> > +        * manually after the DMA completes.
> > +        */
> > +       len = ALIGN(drv_data->len, 4);
> 
> This is actually a property of the DMA controller.

Right and this is not the proper place for this but I wasn't sure where to
put this information...

> struct dma_device already has this:
> 
>  * @copy_align: alignment shift for memcpy operations
>  * @xor_align: alignment shift for xor operations
>  * @pq_align: alignment shift for pq operations
>  * @fill_align: alignment shift for memset operations
> (...)
>         u8 copy_align;
>         u8 xor_align;
>         u8 pq_align;
>         u8 fill_align;
> 
> To align memcpy's on 4 bytes you can e.g. set .copy_align
> to 2.
> 
> So the syntax is number of shifts 1 << 2 = 4.
> 
> If slave transfers can expose the same type of property we should
> maybe introdyce .slave_align in the same manner so you don't have
> to assume the worst in the driver.

... so this sounds good to me.
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