On 08/28/2014 06:46 PM, Tony Lindgren wrote: >> To use DMA you don't have to enable it in SCR register you can also use >> the FCR register. The manual says that you can only write this DMA >> enable bit in the FCR register if the baud clock is not running. And >> guess what: same thing: I only *toggle* the DMA enable bit here (it >> remains 0 later) and the core won't hit idle. >> Same effect if I toggle this bit while the baud clock is running (the >> manual says that this bit can only be written if the baud clock is not >> running). Seems like the UART is following its own specification and it >> remains blocking once the DMA was enabled. >> It would be nice if someone from the UART-IP team could ACK this. > > Sounds like there should be some way to clear that state.. I wonder > if omap-serial.c had something before it's DMA support was removed? Its DMA was removed? Like there was DMA support? > I'd assume when the UART is powered down by runtime PM it's state > is completetely reset and we could restore the non-DMA state? I tried that by canceling the RX-DMA request and removing the DMA-enable bits from UART but it didn't help. Then I noticed that once that DMA en bit is set, the UART won't do any idle. > Maybe post your current patches and a test patch to try to toggle > the DMA on and off? Oh. I pushed my dirty tree to git://git.breakpoint.cc/bigeasy/linux.git uart_v8_wip The top most commit does not setup dma at all and adds commented out code how to enable DMA via SCR or FCR register. With this I hit core-off. Once _one_ of the modes are enabled, it doesn't work anymore. I will try to address review comments tomorrow and hopefully post a v8 based on -rc2. The same goes for your patch (which I will try tomorrow). >> Bah. Does it make sense to use runtime-PM if we can't hit core-off? I'm >> thinking to add a printk once dma is enabled says that runtime-pm is >> switched off. > > Well if we can't find a way to unset the DMA registers in the UART, > how about only enable it if a kernel cmdline option is specified? Hmmm. So removing DMA properties from DT is not an option? Currently I added a message that DMA might forbid low power mode so people might remove "dma-names" from DT (or assign "") and DMA is off. However if this is no good, then I guess I could add kernel module which enables DMA. > > We do have runtime PM working without it, and the serial console > super important for any kind of debugging no matter what idle > mode the SoC hits.. So let's not break that. Yeah, I know. I've been debugging while I broke something which means uart wasn't working :) > > Regards, > > Tony Sebastian -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
