linux-kernel - Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

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Message-ID: <20150807131746.GM7576@n2100.arm.linux.org.uk>
Date:	Fri, 7 Aug 2015 14:17:47 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	nsekhar@...com, linux-omap@...r.kernel.org,
	linux-serial@...r.kernel.org, john.ogness@...utronix.de,
	Peter Ujfalusi <peter.ujfalusi@...com>
Subject: Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic
 transfers

On Fri, Aug 07, 2015 at 02:35:45PM +0200, Sebastian Andrzej Siewior wrote:
> On 08/07/2015 12:55 PM, Russell King - ARM Linux wrote:
> > On Fri, Aug 07, 2015 at 10:41:57AM +0200, Sebastian Andrzej Siewior wrote:
> >> This DMA driver is used by 8250-omap on DRA7-evm. There is one
> >> requirement that is to pause a transfer. This is currently used on the RX
> >> side. It is possible that the UART HW aborted the RX (UART's RX-timeout)
> >> but the DMA controller starts the transfer shortly after.
> >> Before we can manually purge the FIFO we need to pause the transfer,
> >> check how many bytes it already received and terminate the transfer
> >> without it making any progress.
> >>
> >> >From testing on the TX side it seems that it is possible that we invoke
> >> pause once the transfer has completed which is indicated by the missing
> >> CCR_ENABLE bit but before the interrupt has been noticed. In that case the
> >> interrupt will come even after disabling it.
> > 
> > How do you cope with the OMAP DMA hardware clearing its FIFO when you
> > pause it?
> 
> I don't

... and so you introduce a silent data loss bug into the driver.  That's
not very clever.

> Right now the 820-omap (8250-dma in general, too but they don't use
> this driver) pause only the RX transfer in an error condition. This
> means it is only device-to-mem transfer. I only mentioned the TX
> transfer here since this was easier to test.

That may be how 8250 works, but 8250 is not everything.  You can't ignore
this problem.  You have to deal with it - either by not allowing a channel
that would loose data to be paused, or by recovering from that condition.
You're not doing either in your patch.

Therefore, I have no other option but to NAK your change.  Sorry.

Please fix this.

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