Hi,
On Mon, Jun 24, 2024 at 6:31 AM Johan Hovold <johan+linaro@xxxxxxxxxx> wrote:
>
> The Qualcomm GENI serial driver does not handle buffer flushing and used
> to print garbage characters when the circular buffer was cleared. Since
> commit 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo") this
> instead results in a lockup due to qcom_geni_serial_send_chunk_fifo()
> spinning indefinitely in the interrupt handler.
>
> This is easily triggered by interrupting a command such as dmesg in a
> serial console but can also happen when stopping a serial getty on
> reboot.
>
> Fix the immediate issue by printing NUL characters until the current TX
> command has been completed.
>
> Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
> Reported-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Signed-off-by: Johan Hovold <johan+linaro@xxxxxxxxxx>
> ---
> drivers/tty/serial/qcom_geni_serial.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
I don't love this, though it's better than a hard lockup. I will note
that it doesn't exactly restore the old behavior which would have
(most likely) continued to output data that had previously been in the
FIFO but that had been cancelled.
...actually, if we're looking for a short term fix that mimics the old
behavior more closely, what would you think about having a
driver-local buffer that we fill when we kick off the transfer. Then
the data can't go away from underneath us. It's an extra copy, but
it's just a memory-to-memory copy which is much faster than the MMIO
copy we'll eventually need to do anyway... This local buffer would
essentially act as a larger FIFO.
You could choose the local buffer size to balance being able to cancel
quickly vs. using the FIFO efficiently.
-Doug
