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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 08/07/2015 03:17 PM, Russell King - ARM Linux wrote:
> 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.

Would it be okay if I only allow pause for RX-transfers?
For TX-transfers, I would need to update the start-address so the
transfers begins where it stopped. However based on your concern I
can't really assume that the position reported by the HW is the correct
one.

Sebastian