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 -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
