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

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Date:	Fri, 07 Aug 2015 12:36:14 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Peter Ujfalusi <peter.ujfalusi@...com>,
	Vinod Koul <vinod.koul@...el.com>
CC:	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>
Subject: Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

On 08/07/2015 11:44 AM, Peter Ujfalusi wrote:
> On 08/07/2015 11:41 AM, 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.
> 
> with a short testing audio did not broke (the only user of pause/resume)
> Some comments embedded.
> 
>> Cc: <stable@...r.kernel.org>
> 
> Why stable? This is not fixing any bugs since the PAUSE was not allowed for
> non cyclic transfers.

Hmmm. The DRA7x was using pause before for UART. I just did not see it
coming that it was not allowed here. John made a similar change to the
edma driver and I assumed it went stable but now I see that it was just
cherry-picked into the ti tree.
If you are not comfortable it being stable material I can drop it.

>> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>> ---
>>  drivers/dma/omap-dma.c | 54 ++++++++++++++++++++++++++++++++++++++------------
>>  1 file changed, 41 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/dma/omap-dma.c b/drivers/dma/omap-dma.c
>> index 249445c8a4c6..6b8497203caf 100644
>> --- a/drivers/dma/omap-dma.c
>> +++ b/drivers/dma/omap-dma.c
>> @@ -299,7 +299,7 @@ static void omap_dma_start(struct omap_chan *c, struct omap_desc *d)
>>  	omap_dma_chan_write(c, CCR, d->ccr | CCR_ENABLE);
>>  }
>>  
>> -static void omap_dma_stop(struct omap_chan *c)
>> +static int omap_dma_stop(struct omap_chan *c)
>>  {
>>  	struct omap_dmadev *od = to_omap_dma_dev(c->vc.chan.device);
>>  	uint32_t val;
>> @@ -342,8 +342,26 @@ static void omap_dma_stop(struct omap_chan *c)
>>  
>>  		omap_dma_glbl_write(od, OCP_SYSCONFIG, sysconfig);
>>  	} else {
>> +		int i = 0;
>> +
>> +		if (!(val & CCR_ENABLE))
>> +			return -EINVAL;
>> +
>>  		val &= ~CCR_ENABLE;
>>  		omap_dma_chan_write(c, CCR, val);
>> +		do {
>> +			val = omap_dma_chan_read(c, CCR);
>> +			if (!(val & (CCR_RD_ACTIVE | CCR_WR_ACTIVE)))
>> +				break;
>> +			if (i > 100)
> 
> if (++i > 100)
> 	break;
> to avoid infinite loop?

Ah. So I forgot to increment the counter. A few lines above there is
the same loop as a workaround for something. This is the same loop. I
could merge the loop + warning if you prefer. to have those things in
one place. I could also just increment i. Merging the two loops might
be better.

>> +				break;
>> +			udelay(5);
>> +		} while (1);
>> +
>> +		if (val & (CCR_RD_ACTIVE | CCR_WR_ACTIVE))
> 
> if (i > 100) ?

While that would work, too I think it is more explicit to the reader if
you check for the condition that is important to you.

>> +			dev_err(c->vc.chan.device->dev,
>> +				"DMA drain did not complete on lch %d\n",
>> +				c->dma_ch);
>>  	}
>>  
>>  	mb();

Sebastian

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