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

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Message-ID: <20180517041946.GQ13271@vkoul-mobl>
Date:   Thu, 17 May 2018 09:49:46 +0530
From:   Vinod Koul <vinod.koul@...aro.org>
To:     Frank Mori Hess <fmh6jj@...il.com>
Cc:     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 15-05-18, 11:50, Frank Mori Hess wrote:
> On Tue, May 15, 2018 at 2:21 AM, Vinod <vkoul@...nel.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

no data loss
> read residue

here as well
> dma terminate

Oh yes, we aborted...
> 
> 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.

As I said terminate is abort. It cleans up the channel and is supposed to
used for cleanup not for stopping. See Documentation:

- device_terminate_all

  - Aborts all the pending and ongoing transfers on the channel

  - For aborted transfers the complete callback should not be called

  - Can be called from atomic context or from within a complete
    callback of a descriptor. Must not sleep. Drivers must be able
    to handle this correctly.

  - Termination may be asynchronous. The driver does not have to
    wait until the currently active transfer has completely stopped.
    See device_synchronize.

Thanks
-- 
~Vinod