From: 1=?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= <mans@mansr.com>
To: Sebastian Frias <sf84@laposte.net>
Cc: Vinod Koul <vinod.koul@intel.com>, Mason <slash.tmp@free.fr>,
        Russell King <linux@arm.linux.org.uk>, dmaengine@vger.kernel.org,
        Linus Walleij <linus.walleij@linaro.org>,
        Dan Williams <dan.j.williams@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Jon Mason <jdmason@kudzu.us>, Mark Brown <broonie@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>, Lee Jones <lee.jones@linaro.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Maxime Ripard <maxime.ripard@free-electrons.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Peter Ujfalusi <peter.ujfalusi@ti.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Thibaud Cornic <thibaud_cornic@sigmadesigns.com>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished
In-Reply-To: <6ce1ea97-1d68-2203-c7b4-73315e801655@laposte.net> (Sebastian
        Frias's message of "Fri, 9 Dec 2016 11:25:57 +0100")
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Date: Fri, 09 Dec 2016 11:35:20 +0000
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Sebastian Frias <sf84@laposte.net> writes:

> On 09/12/16 07:59, Vinod Koul wrote:
>> On Thu, Dec 08, 2016 at 04:48:18PM +0000, M?ns Rullg?rd wrote:
>>> Vinod Koul <vinod.koul@intel.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.

-- 
M?ns Rullg?rd