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

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Date:   Thu, 8 Dec 2016 13:31:34 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Måns Rullgård <mans@...sr.com>
Cc:     Vinod Koul <vinod.koul@...el.com>, Mason <slash.tmp@...e.fr>,
        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

On Thu, Dec 8, 2016 at 1:20 PM, Måns Rullgård <mans@...sr.com> wrote:
> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
>> On Thu, Dec 8, 2016 at 12:44 PM, Måns Rullgård <mans@...sr.com> wrote:
>>> Vinod Koul <vinod.koul@...el.com> writes:
>>>> 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 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@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds