hw/char: sifive_uart: Raise IRQ according to the Tx/Rx watermark thresholds

hw/char: sifive_uart: Raise IRQ according to the Tx/Rx watermark thresholds

Currently, the SiFive UART raises an IRQ whenever:

  1. ie.txwm is enabled.
  2. ie.rxwm is enabled and the Rx FIFO is not empty.

It does not check the watermark thresholds set by software. However,
since commit [1] changed the SiFive UART character printing from
synchronous to asynchronous, Tx overflows may occur, causing characters
to be dropped when running Linux because:

  1. The Linux SiFive UART driver sets the transmit watermark level to 1
     [2], meaning a transmit watermark interrupt is raised whenever a
     character is enqueued into the Tx FIFO.
  2. Upon receiving a transmit watermark interrupt, the Linux driver
     transfers up to a full Tx FIFO's worth of characters from the Linux
     serial transmit buffer [3], without checking the txdata.full flag
     before transferring multiple characters [4].

To fix this issue, we must honor the Tx/Rx watermark thresholds and
raise interrupts only when the Tx threshold is exceeded or the Rx
threshold is undercut.

[1] 53c1557b230986ab6320a58e1b2c26216ecd86d5
[2] https://github.com/torvalds/linux/blob/master/drivers/tty/serial/sifive.c#L1039
[3] https://github.com/torvalds/linux/blob/master/drivers/tty/serial/sifive.c#L538
[4] https://github.com/torvalds/linux/blob/master/drivers/tty/serial/sifive.c#L291

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Signed-off-by: Emmanuel Blot <emmanuel.blot@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250911160647.5710-2-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>