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

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Message-ID: <yw1xfulyzj2b.fsf@unicorn.mansr.com>
Date:   Thu, 08 Dec 2016 12:44:44 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Mason <slash.tmp@...e.fr>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        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>,
        Thomas Gambier <thomas_gambier@...madesigns.com>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished

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

> On 08/12/2016 13:20, Måns Rullgård wrote:
>
>> The only problem we have is that nobody envisioned hardware where the
>> dma engine indicates completion slightly too soon.  I suspect there's a
>> fifo or such somewhere, and the interrupt is triggered when the last
>> byte has been placed in the fifo rather than when it has been removed
>> which would have been more correct.
>
> As I (tried to) explain here:
> https://marc.info/?l=dmaengine&m=148007808418242&w=2
>
> A *read* MBUS agent raises its IRQ when it is safe for the memory
> to be overwritten (i.e. every byte has been pushed into the pipe).
>
> A *write* MBUS agent raises its IRQ when it is safe for another
> agent to read any one of the transferred bytes.
>
> The issue comes from the fact that, for a memory-to-device transfer,
> the system will receive the read agent's IRQ, but most devices
> (NFC, SATA) don't have an IRQ line to signal that their part of the
> operation is complete.

SATA does, actually.  Nevertheless, it's an unusual design.

-- 
Måns Rullgård