Tearing down DMA transfer setup after DMA client has finished

Geert Uytterhoeven [off-list ref] writes:

Hi M?ns,

On Thu, Dec 8, 2016 at 12:47 PM, M?ns Rullg?rd [off-list ref] wrote:

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Geert Uytterhoeven [off-list ref] writes:

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On Thu, Dec 8, 2016 at 11:54 AM, Mason [off-list ref] wrote:

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On 08/12/2016 11:39, Vinod Koul wrote:

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On Wed, Dec 07, 2016 at 04:45:58PM +0000, M?ns Rullg?rd wrote:

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Vinod Koul [off-list ref] writes:

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On Tue, Dec 06, 2016 at 01:14:20PM +0000, M?ns Rullg?rd wrote:

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That's not going to work very well.  Device drivers typically request
dma channels in their probe functions or when the device is opened.
This means that reserving one of the few channels there will inevitably
make some other device fail to operate.

No that doesn't make sense at all, you should get a channel only when you
want to use it and not in probe!

Tell that to just about every single driver ever written.

Not really, few do yes which is wrong but not _all_ do that.

Vinod,

Could you explain something to me in layman's terms?

I have a NAND Flash Controller driver that depends on the
DMA driver under discussion.

Suppose I move the dma_request_chan() call from the driver's
probe function, to the actual DMA transfer function.

I would want dma_request_chan() to put the calling thread
to sleep until a channel becomes available (possibly with
a timeout value).

But Maxime told me dma_request_chan() will just return
-EBUSY if no channels are available.

Am I supposed to busy wait in my driver's DMA function
until a channel becomes available?

Can you fall back to PIO if requesting a channel fails?

Alternatively, dma_request_chan() could always succeed, and
dmaengine_prep_slave_sg() could fail if the channel is currently not
available due to a limitation on the number of active channels, and
the driver could fall back to PIO for that transfer.

Why are we debating this nonsense?  There is an easy fix that doesn't
require changing the semantics of existing functions or falling back to
slow pio.

You still want to fall back to PIO if the DMA engine is not available at all
(e.g. DMA engine driver not compiled in, or module not loaded).

That's a choice for each device driver to make.  Some devices don't have
a pio mode at all.

-- 
M?ns Rullg?rd