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From:   Frank Mori Hess <fmh6jj@gmail.com>
Date:   Tue, 15 May 2018 11:50:46 -0400
Message-ID: <CAJz5Open++Dj=a4ty-JbnZBFoqbMQKU6qYumJKSRMRLp_3erSw@mail.gmail.com>
Subject: Re: Revert "dmaengine: pl330: add DMA_PAUSE feature"
To:     Vinod <vkoul@kernel.org>
Cc:     Marek Szyprowski <m.szyprowski@samsung.com>,
        dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org,
        Dan Williams <dan.j.williams@intel.com>,
        r.baldyga@hackerion.com, Krzysztof Kozlowski <krzk@kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Linux Samsung SOC <linux-samsung-soc@vger.kernel.org>
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On Tue, May 15, 2018 at 2:21 AM, Vinod <vkoul@kernel.org> wrote:
>
> For Pause/resume data loss is _not_ expected.
>
>> > and some of the 8250 drivers like 8250_dw.c set a maxburst > 1.  If it
>> > can't count on the pause/residue/terminate working without data loss
>> > then it is just broken.  As is the 8250_omap.c driver.  Is the
>> > description of dmaengine_pause in the documentation of "This pauses
>> > activity on the DMA channel without data loss" to be interpreted as
>> > "as long as you resume afterwards"?
>>
>> I assume that this requirement is for both - resuming and terminating.
>
> Terminate is abort, data loss may happen here.

Wait, are you saying if you do

dma pause
read residue
dma terminate

then it is acceptable for there to be data loss, because it can be
blamed on the terminate?  In that case the usage of dmaengine in all
the 8250 serial drivers is broken.  Every time there is a break in the
incoming rx data, the 8250 driver does pause/residue/terminate which
could potentially cause data from the incoming data stream to be
dropped.