Re: Tearing down DMA transfer setup after DMA client has finished

[Geert Uytterhoeven]

On Thu, Dec 8, 2016 at 1:20 PM, MÃns RullgÃrd <mans@xxxxxxxxx> wrote:
> Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> writes:
>> On Thu, Dec 8, 2016 at 12:44 PM, MÃns RullgÃrd <mans@xxxxxxxxx> wrote:
>>> Vinod Koul <vinod.koul@xxxxxxxxx> writes:
>>>> On Wed, Dec 07, 2016 at 04:45:58PM +0000, MÃns RullgÃrd wrote:
>>>>> Vinod Koul <vinod.koul@xxxxxxxxx> writes:
>>>>> > On Tue, Dec 06, 2016 at 01:14:20PM +0000, MÃns RullgÃrd 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 doesnt 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.
>>>
>>> Every driver I ever looked at does.  Name one you consider "correct."
>>
>> I'm far from claiming that drivers/tty/serial/sh-sci.c is perfect, but
>> it does request DMA channels at open time, not at probe time.
>
> In the part quoted above, I said most drivers request dma channels in
> their probe or open functions.  For the purposes of this discussion,
> that distinction is irrelevant.  In either case, the channel is held
> indefinitely.  If this wasn't the correct way to use the dmaengine,
> there would be no need for the virt-dma helpers which are specifically
> designed for cases the one currently at hand.

Sorry, I mainly read Vinod's "not in probe", and missed your "or when the
device is opened".

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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                                -- Linus Torvalds