Re: [PATCH v2 6/7] serial: qcom-geni: Fix suspend while active UART xfer

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Thu, May 30, 2024 at 03:45:58PM -0700, Douglas Anderson wrote:
> On devices using Qualcomm's GENI UART it is possible to get the UART
> stuck such that it no longer outputs data. Specifically, I could
> reproduce this problem by logging in via an agetty on the debug serial
> port (which was _not_ used for kernel console) and running:
>   cat /var/log/messages
> ...and then (via an SSH session) forcing a few suspend/resume cycles.
> 
> Digging into this showed a number of problems that are all related.
> 
> The root of the problems was with qcom_geni_serial_stop_tx_fifo()
> which is called as part of the suspend process. Specific problems with
> that function:
> - When we cancel an in-progress "tx" command it doesn't appear to
>   fully drain the FIFO. That meant qcom_geni_serial_tx_empty()
>   continued to report that the FIFO wasn't empty. The
>   qcom_geni_serial_start_tx_fifo() function didn't re-enable
>   interrupts in this case so we'd never start transferring again.
> - We cancelled the current "tx" command but we forgot to zero out
>   "tx_remaining". This confused logic elsewhere in the driver
> - From experimentation, it appears that cancelling the "tx" command
>   could drop some of the queued up bytes. While maybe not the end of
>   the world, it doesn't seem like we should be dropping bytes when
>   stopping the FIFO, which is defined more of a "pause".
> 
> One idea to fix the above would be to add FIFO draining to
> qcom_geni_serial_stop_tx_fifo(). However, digging into the
> documentation in serial_core.h for stop_tx() makes this seem like the
> wrong choice. Specifically stop_tx() is called with local interrupts
> disabled. Waiting for a FIFO (which might be 64 bytes big) to drain at
> 115.2 kbps doesn't seem like a wise move.
> 
> Ideally qcom_geni_serial_stop_tx_fifo() would be able to pause the
> transmitter, but nothing in the documentation for the GENI UART makes
> me believe that is possible.
> 
> Given the lack of better choices, we'll change
> qcom_geni_serial_stop_tx_fifo() to simply disable the
> TX_FIFO_WATERMARK interrupt and call it a day. This seems OK as per
> the serial core docs since stop_tx() is supposed to stop transferring
> bytes "as soon as possible" and there doesn't seem to be any possible
> way to stop transferring sooner. As part of this, get rid of some of
> the extra conditions on qcom_geni_serial_start_tx_fifo() which simply
> weren't needed and are now getting in the way. It's always fine to
> turn the interrupts on if we want to receive and it'll be up to the
> IRQ handler to turn them back off if somehow they're not needed. This
> works fine.
> 
> Unfortunately, doing just the above change causes new/different
> problems with suspend/resume. Now if you suspend while an active
> transfer is happening you can find that after resume time you're no
> longer receiving UART interrupts at all. It appears to be important to
> drain the FIFO and send a "cancel" command if the UART is active to
> avoid this. Since we've already decided that
> qcom_geni_serial_stop_tx_fifo() shouldn't be doing this, let's add the
> draining / cancelling logic to the shutdown() call where it should be
> OK to delay a bit. This is called as part of the suspend process via
> uart_suspend_port().
> 
> Finally, with all of the above, the test case where we're spamming the
> UART with data and going through suspend/resume cycles doesn't kill
> the UART and doesn't drop bytes.
> 
> NOTE: though I haven't gone back and validated on ancient code, it
> appears from code inspection that many of these problems have existed
> since the start of the driver. In the very least, I could reproduce
> the problems on vanilla v5.15. The problems don't seem to reproduce
> when using the serial port for kernel console output and also don't
> seem to reproduce if nothing is being printed to the console at
> suspend time, so this is presumably why they were not noticed until
> now.

...

> +	qcom_geni_serial_poll_bitfield(uport, SE_GENI_M_GP_LENGTH, 0xffffffff,

It's easy to miscount f:s, GENMASK()?

> +				       port->tx_total - port->tx_remaining);

-- 
With Best Regards,
Andy Shevchenko