linux-kernel - Re: Tearing down DMA transfer setup after DMA client has finished

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Message-ID: <yw1xoa0mzk5e.fsf@unicorn.mansr.com>
Date:   Thu, 08 Dec 2016 12:21:17 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Mason <slash.tmp@...e.fr>, Vinod Koul <vinod.koul@...el.com>,
        Russell King <linux@....linux.org.uk>,
        dmaengine@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Dan Williams <dan.j.williams@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Jon Mason <jdmason@...zu.us>, Mark Brown <broonie@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Lee Jones <lee.jones@...aro.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Arnd Bergmann <arnd@...db.de>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Sebastian Frias <sf84@...oste.net>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

> Hi Måns,
>
> On Thu, Dec 8, 2016 at 12:47 PM, Måns Rullgård <mans@...sr.com> wrote:
>> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
>>> On Thu, Dec 8, 2016 at 11:54 AM, Mason <slash.tmp@...e.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@...el.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.
>>
>> 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.

-- 
Måns Rullgård