On Mon, Mar 23, 2020 at 04:50:17PM +0300, Sergey Semin wrote: > On Mon, Mar 23, 2020 at 11:01:09AM +0100, Maxime Ripard wrote: > > On Mon, Mar 23, 2020 at 05:46:09AM +0300, Sergey.Semin@xxxxxxxxxxxxxxxxxxxx wrote: > > > From: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx> > > > > > > There are races possible in the dw8250_set_termios() callback method > > > and while the device is in PM suspend state. A race condition may > > > happen if the baudrate clock source device is shared with some other > > > device (in our machine it's another DW UART port). In this case if that > > > device changes the clock rate while serial console is using it the > > > DW 8250 UART port might not only end up with an invalid uartclk value > > > saved, but may also experience a distorted output data since baud-clock > > > could have been changed. In order to fix this lets enable an exclusive > > > reference clock rate access in case if "baudclk" device is specified. > > > > > > So if some other device also acquires the rate exclusivity during the > > > time of a DW UART 8250 port being opened, then DW UART 8250 driver > > > won't be able to alter the baud-clock. It shall just use the available > > > clock rate. Similarly another device also won't manage to change the > > > rate at that time. If nothing else have the exclusive rate access > > > acquired except DW UART 8250 driver, then the driver will be able to > > > alter the rate as much as it needs to in accordance with the currently > > > implemented logic. > > clk_rate_exclusive_get is pretty intrusive, and due to the usual > > topology of clock trees, this will lock down 3-4 parent clocks to > > their current rate as well. In the Allwinner SoCs case for example, > > this will lock down the same PLL than the one used by the CPU, > > preventing cpufreq from running. > > Speaking about weak design of a SoC' clock tree. Our problems are nothing > with respect to the Allwinner SoC, in which case of changing the > CPU-frequency may cause the UART glitches subsequently causing data > transfer artefacts.) Moreover as I can see the same issue may raise for > I2C, QSPI, PWM devices there. > > Anyway your concern does make sense. > > > However, the 8250 has a pretty wide range of dividers and can adapt to > > any reasonable parent clock rate, so we don't really need to lock the > > rate either, we can simply react to a parent clock rate change using > > the clock notifiers, just like the SiFive UART is doing. > > > > I tried to do that, but given that I don't really have an extensive > > knowledge of the 8250, I couldn't find a way to stop the TX of chars > > while we change the clock rate. I'm not sure if this is a big deal or > > not, the SiFive UART doesn't seem to care. > > Yes, your solution is also possible, but even in case of stopping Tx/Rx it > doesn't lack drawbacks. First of all AFAIK there is no easy way to just > pause the transfers. We'd have to first wait for the current transfers > to be completed, then somehow lock the port usage (both Tx and Rx > traffic), permit the reference clock rate change, accordingly adjust the > UART clock divider, and finally unlock the port. While if we don't mind > to occasionally have UART data glitches, we can just adjust the UART ref > divider synchronously with ref clock rate change as you and SiFive UART > driver suggest. > > So we are now at a zugzwang - a fork to three not that good solutions: > 1) lock the whole clock branch and provide a glitchless interfaces. But > by doing so we may (in case of Allwinner SoCs we will) lockup some very > important functionality like CPU-frequency change while the UART port is > started up. In this case we won't have the data glitches. > 2) just adjust the UART clock divider in case of reference clock rate > change (use the SiFive UART driver approach). In this case we may have the > data corruption. > 3) somehow implement the algo: wait for the transfers to be completed, > lock UART interface (it's possible for Tx, but for Rx in case of no handshake > enabled it's simply impossible), permit the ref clock rate change, > adjust the UART divider, then unlock the UART interface. In this case the data > glitches still may happen (if no modem control is available or > handshakes are disabled). > > As for the cases of Baikal-T1 UARTs the first solutions is the most suitable. > We don't lock anything valuable, since a base PLL output isn't directly > connected to any device and it's rate once setup isn't changed during the > system running. On the other hand I don't mind to implement the second > solution, even though it's prone to data glitches. Regarding the solution > 3) I won't even try. It's too complicated, I don't have time and > test-infrastructure for this. > > So Andy what do you think? >From Intel HW perspective the first two are okay, but since Maxime is against first, you have the only option from your list. Perhaps somebody may give option 4) here... -- With Best Regards, Andy Shevchenko
