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

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Message-ID: <yw1xoa0lxrlz.fsf@unicorn.mansr.com>
Date:   Fri, 09 Dec 2016 11:35:20 +0000
From:   1Måns Rullgård <mans@...sr.com>
To:     Sebastian Frias <sf84@...oste.net>
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>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished

Sebastian Frias <sf84@...oste.net> writes:

> On 09/12/16 07:59, Vinod Koul wrote:
>> On Thu, Dec 08, 2016 at 04:48:18PM +0000, Måns Rullgård wrote:
>>> Vinod Koul <vinod.koul@...el.com> writes:
>>>
>>>> To make it efficient, disregarding your Sbox HW issue, the solution is
>>>> virtual channels. You can delink physical channels and virtual channels. If
>>>> one has SW controlled MUX then a channel can service any client. For few
>>>> controllers request lines are hard wired so they cant use any channel. But
>>>> if you dont have this restriction then driver can queue up many transactions
>>>> from different controllers.
>>>
>>> Have you been paying attention at all?  This exactly what the driver
>>> ALREADY DOES.
>> 
>> And have you read what the question was?

I wrote the driver.  I think I know what Mason and I are asking.

> I think many people appreciate the quick turn around time and
> responsiveness of knowledgeable people making constructive remarks in
> this thread, but it looks we are slowly drifting away from the main
> problem.
>
> If we had to sum up the discussion, the current DMA API/framework in
> Linux seems to lack a way to properly handle this HW (or if it has a
> way, the information got lost somewhere).
>
> What concrete solution do you propose?
>
> Alternatively, one can think of the current issue (i.e.: the fact that
> the IRQ arrives "too soon") in a different way.  Instead of thinking
> the IRQ indicates "transfer complete", it is indicating "ready to
> accept another command", which in practice (and with proper API
> support) can translate into efficient queuing of DMA operations.

For multiple back to back transfers to the same peripheral, it is indeed
a slight optimisation.  What's apparently lacking is some way of doing a
full flush at the end of a series of transfers, before switching the
channel to another device.

-- 
Måns Rullgård