Re: [RFC] dmaengine: add new api for preparing simple slave transfer

[Linus Walleij <linus.walleij@linaro.org>]

On Fri, Jun 10, 2011 at 3:33 PM, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Fri, Jun 10, 2011 at 05:18:46PM +0530, Raju, Sundaram wrote:
>> Now DMACs capable of 3D transfer, do transfer of the whole 1D
>> buffer per sync received or even whole 2D buffer per sync received
>> (based on the sync rate programmed in the DMAC).
>
> The only issue which I see that we don't cover is the case where you want
> to describe a single buffer which is organised as N bytes to be transferred,
> M following bytes to be skipped, N bytes to be transferred, M bytes to be
> skipped.  I doubt there are many controllers which can be programmed with
> both 'N' and 'M' parameters directly.

Sundaram is this how your controller works?
I mean the hardware can skip over sequences like this?

When we added the config interface to DMAengine I originally included
a "custom config" call, but Dan wanted me to keep it out until we
had some specific usecase for it. FSLDMA recently started
to use it.

Notice how dmaengine_slave_config() is implemented:

static inline int dmaengine_slave_config(struct dma_chan *chan,
					  struct dma_slave_config *config)
{
	return dmaengine_device_control(chan, DMA_SLAVE_CONFIG,
			(unsigned long)config);
}

So what is passed to the driver is just an unsigned long.

This is actually modeled to be ioctl()-like so you can pass in a
custom config to the same callback on the device driver,
just use some other enumerator than DMA_SLAVE_CONFIG,
say like FSLDMA already does with FSLDMA_EXTERNAL_START.

Just put some enumerator in enum dma_ctrl_cmd in
dmaengine.h such as SDMA_TEXAS_STRIDE_CONFIG and call
like this:

/* However that config struct needs to look, basically */
static struct sdma_ti_stride_cgf = {
     take = M,
     skip = N,
};

ret = chan->device->device_control(chan, SDMA_TEXAS_STRIDE_CONFIG,
&sdma_ti_stride_cfg);

Or something like this.

Thanks,
Linus Walleij