On Sat, 2023-10-14 at 12:13 +0100, Richard Purdie via lists.openembedded.org wrote: > On Sat, 2023-10-14 at 10:41 +0100, Richard Purdie wrote: > > Brief summary: > > > > We're seeing an issue on x86_64 with 6.5.X where data appears to be > > left in the transmission buffer and not sent to the port on the second > > serial port (ttyS1) until we trigger it with intervention. > > > > Paul Gortmaker did some painful bisection over a few days down to: > > > > serial: core: Start managing serial controllers to enable runtime PM > > https://lore.kernel.org/linux-serial/1431f5b4-fb39-483b-9314-ed2b7c118...@gmail.com/T/#t > > Having poked around a bit and knowing nothing about any of this, should > this bit of new code added in the above commit to __uart_start() in > serial_core.c: > > /* > * Start TX if enabled, and kick runtime PM. If the device is not > * enabled, serial_port_runtime_resume() calls start_tx() > again > * after enabling the device. > */ > if (pm_runtime_active(&port_dev->dev)) > port->ops->start_tx(port); > > > actually be something like: > > > if (pm_runtime_active(&port_dev->dev) || > !pm_runtime_enabled(&port_dev->dev)) > port->ops->start_tx(port); > > > since there are uarts that don't enable runtime PM? > > I notice that 16550A I'm using doesn't set UART_CAP_RPM and since we > have data left in the xmit buffer (I managed to confirm that), it is as > if during init, there is a race between the serial probing and the > getty putting data in the buffer? If it weren't statrted, that would > explain things...
The above change didn't work but what does appear to be making a difference is making this code call start_tx unconditionally which is what it did prior to the patch. That does cause a "wake" when there might not be any data but the code handles that gracefully. I therefore suspect this is the place the issue is, the question is what the right conditions for calling start_tx are? I'll keep going with testing of that as the intermittent nature does make this hard to know if any change helps or not. Cheers, Richard
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