Tearing down DMA transfer setup after DMA client has finished

[Geert Uytterhoeven]

On Thu, Dec 8, 2016 at 11:54 AM, Mason <slash.tmp at free.fr> wrote:
> On 08/12/2016 11:39, Vinod Koul wrote:
>> On Wed, Dec 07, 2016 at 04:45:58PM +0000, Måns Rullgård wrote:
>>> Vinod Koul <vinod.koul at intel.com> writes:
>>>> On Tue, Dec 06, 2016 at 01:14:20PM +0000, Måns Rullgård wrote:
>>>>> 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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

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