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

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Date:	Fri, 7 Aug 2015 10:55:02 -0700
From:	Greg KH <greg@...ah.com>
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,
	Russell King <rmk+kernel@....linux.org.uk>,
	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 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.
> 
> The AM572x manual says that we have to wait for the CCR_RD_ACTIVE &
> CCR_WR_ACTIVE bits to be gone before programming it again here is the
> drain loop. Also it looks like without the drain the TX-transfer makes
> sometimes progress.
> 
> One note: The pause + resume combo is broken because after resume the
> the complete transfer will be programmed again. That means the already
> transferred bytes (until the pause event) will be sent again. This is
> currently not important for my UART user because it does only pause +
> terminate.
> 
> Cc: <stable@...r.kernel.org>

You don't get a "add support" patch into the stable tree unless it's a
trivial device id or quirk table addition, please go re-read
Documentation/stable_kernel_rules.txt

thanks,

greg k-h
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