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

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Message-ID: <yw1xoa137fvr.fsf@unicorn.mansr.com>
Date:   Fri, 25 Nov 2016 15:12:24 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Mason <slash.tmp@...e.fr>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Vinod Koul <vinod.koul@...el.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Dave Jiang <dave.jiang@...el.com>,
        Arnd Bergmann <arnd@...db.de>, Mark Brown <broonie@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        dmaengine@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
        Jon Mason <jon.mason@...el.com>,
        Lee Jones <lee.jones@...aro.org>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished

Mason <slash.tmp@...e.fr> writes:

> On 25/11/2016 15:17, Russell King - ARM Linux wrote:
>> On Fri, Nov 25, 2016 at 02:03:20PM +0000, Måns Rullgård wrote:
>>> Russell King - ARM Linux <linux@...linux.org.uk> writes:
>>>
>>>> On Fri, Nov 25, 2016 at 01:50:35PM +0000, Måns Rullgård wrote:
>>>>> Russell King - ARM Linux <linux@...linux.org.uk> writes:
>>>>>> It would be unfair to augment the API and add the burden on everyone
>>>>>> for the new API when 99.999% of the world doesn't require it.
>>>>>
>>>>> I don't think making this particular dma driver wait for the descriptor
>>>>> callback to return before reusing a channel quite amounts to a horrid
>>>>> hack.  It certainly wouldn't burden anyone other than the poor drivers
>>>>> for devices connected to it, all of which are specific to Sigma AFAIK.
>>>>
>>>> Except when you stop to think that delaying in a tasklet is exactly
>>>> the same as randomly delaying in an interrupt handler - the tasklet
>>>> runs on the return path back to the parent context of an interrupt
>>>> handler.  Even if you sleep in the tasklet, you're sleeping on behalf
>>>> of the currently executing thread - if it's a RT thread, you effectively
>>>> destroy the RT-ness of the thread.  Let's hope no one cares about RT
>>>> performance on that hardware...
>>>
>>> That's why I suggested to do this only if the needed delay is known to
>>> be no more than a few bus cycles.  The completion callback is currently
>>> the only post-transfer interaction we have between the dma and device
>>> drivers.  To handle an arbitrarily long delay, some new interface will
>>> be required.
>> 
>> And now we're back at the point I made a few emails ago about undue
>> burden which is just about quoted above...
>
> I've had several talks with the HW dev, and I don't think they
> anticipated the need to mux the 3 channels. In their minds,
> customers would choose at most 3 devices to support, and
> assign one channel to each device statically.
>
> In fact, in tango4, supported devices are:
> A) NAND Flash controllers 0 and 1
> NB: the upstream driver only uses controller 0
> B) IDE or SATA controllers 0 and 1
> C) a few crypto HW blocks which do not work as expected (unused)
>
> Customers typically use 1 channel for NAND, maybe 1 for SATA,
> and 1 channel remains unused.

The hardware has two sata controllers, and I have a board that uses both.

-- 
Måns Rullgård