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

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Date:	Thu, 17 Jan 2013 10:48:15 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Vinod Koul <vinod.koul@...el.com>, Dan Williams <djbw@...com>
Cc:	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 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.

> +       /*
> +        * 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.

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.

Vinod/Dan? What do you say about this?

Yours,
Linus Walleij
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