linux-kernel - Re: Revert "dmaengine: pl330: add DMA

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Message-ID: <CAJz5OpcnK22PqaRNERL79kQ6y8ffmpkUSTbcq8HLaexNZfwxdQ@mail.gmail.com>
Date:   Fri, 18 May 2018 15:01:41 -0400
From:   Frank Mori Hess <fmh6jj@...il.com>
To:     Lars-Peter Clausen <lars@...afoo.de>
Cc:     Vinod Koul <vinod.koul@...aro.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        r.baldyga@...kerion.com, Krzysztof Kozlowski <krzk@...nel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Linux Samsung SOC <linux-samsung-soc@...r.kernel.org>
Subject: Re: Revert "dmaengine: pl330: add DMA_PAUSE feature"

On Thu, May 17, 2018 at 1:22 PM, Lars-Peter Clausen <lars@...afoo.de> wrote:
> On 05/17/2018 06:20 PM, Frank Mori Hess wrote:
> The problem is not so much on the software side but even more so on the
> hardware side. Not all hardware even supports aborting a transfer with no
> data loss because there is no precise measurement of how much data has been
> transferred. The residue that is reported is always the lower bound, this
> much has been transferred but it might actually have been more.

I'd just like to point out, if the pl330 driver actually did report a
upper bound on the residue (lower bound on bytes transferred) that
would also blow up Marek's samsung serial driver use case.  Instead of
being in a situation where data loss might occur rarely due to a race
condition, they would be in a situation where data loss occurs every
time they stop a transfer.  Not that such a residue reporting would be
incorrect though, the pl330 driver doesn't even advertise burst level
granularity with its residue reporting.