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

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Date:   Tue, 22 May 2018 09:07:51 +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 21-05-18, 20:56, Frank Mori Hess wrote:
> On Mon, May 21, 2018 at 5:16 AM, Vinod Koul <vinod.koul@...aro.org> wrote:
> >>
> >> I understand the residue can't be read after terminate, that's why
> >> reading the residue is step 2 in pause/residue/terminate.  My question
> >> was whether the entire sequence pause/residue/terminate taken as a
> >> whole can or cannot lose data.  Saying that individual steps can or
> >> can't lose data is not enough, context is required.  The key point is
> >> whether pause flushes in-flight data to its destination or not.  If it
> >> does, and our residue is accurate, the terminate cannot cause data
> >> loss.  If pause doesn't flush, an additional step of flush_sync as
> >> Lars suggested is required.  So pause/flush_sync/residue/terminate
> >> would be the safe sequence that cannot lose data.
> >
> > I wouldn't use cannot, shouldn't would be better here as it depends on HW and
> > where all data has been buffered and if it can be flushed or not.
> >
> > Have you checked if pl330 supports such flushing?
> 
> It does not, at least in the context of pausing.  The dma-330 DMAEND
> instruction flushes in-flight data to its destination, and there are
> read/write barrier instructions also, but none of them can be injected
> on the fly into a running dma thread.  DMAKILL can be, but it discards
> in-flight data.  Currently, if an 8250 serial driver uses the pl330
> for rx dma, the result is possible data loss/corruption.  If there was
> a stronger pause capability, call it "cmd_sync_pause" which guaranteed
> flushing of in-flight data to its destination and accurate residue
> reading when paused, then the 8250 serial driver could check for
> "cmd_sync_pause" and reject dma drivers that do not have that
> capability.  pl330.c would not advertise cmd_sync_pause.  I don't know
> if other dmaengine hardware would be able to support cmd_sync_pause or
> not, I'm mostly just familiar with the pl330.  The ep93xx_dma.c driver
> for example has a m2p_hw_synchronize function which seems to do a
> flush.

Well looks like even adding support for sync_pause doesn't solve your issue on
pl330. Do you want to move this to PIO mode then..?

-- 
~Vinod