Fix SiFive UART spurious IRQ issue and misc updates

[PATCH 0/4] Fix SiFive UART spurious IRQ issue and misc updates

Posted by frank.chang@sifive.com 2 months ago

From: Frank Chang <frank.chang@sifive.com>

This patch series fixes the spurious IRQ issue in the SiFive UART on Linux
introduced after commit [1], which changed character transmission from
synchronous to asynchronous.

Currently, the txwm interrupt pending status is only updated when the
asynchronous transmit handler runs. This can cause the txwm interrupt
state to become unsynchronized between the SiFive UART and the interrupt
controller.

This issue has been observed on resource-constrained systems, where
Linux reports spurious IRQ errors. In these cases, the asynchronous
transmit handler is unable to drain the TX FIFO quickly enough to update
the txwm pending status before software reads the IP register, which
derives the txwm pending state directly from the actual number of
characters in the TX FIFO.

The remaining patches contain miscellaneous updates, including
implementing txctrl.txen and rxctrl.rxen, update IRQ when rxctrl is
written and remove the unused ip variable.

[1] 53c1557b230986ab6320a58e1b2c26216ecd86d5

Frank Chang (4):
  hw/char: sifive_uart: Implement txctrl.txen and rxctrl.rxen
  hw/char: sifive_uart: Sync txwm interrupt pending status after TX FIFO
    enqueue
  hw/char: sifive_uart: Update IRQ when rxctrl is written
  hw/char: sifive_uart: Remove ip variable

 hw/char/sifive_uart.c         | 46 ++++++++++++++++++++++++++++-------
 include/hw/char/sifive_uart.h |  3 ++-
 2 files changed, 39 insertions(+), 10 deletions(-)

--
2.43.0

Re: [PATCH 0/4] Fix SiFive UART spurious IRQ issue and misc updates

Posted by Frank Chang 2 weeks, 6 days ago

Sorry, the title should be: "[PATCH v2 0/4] Fix SiFive UART spurious IRQ
issue and misc updates" instead.

ping

Regards,
Frank Chang

On Wed, Dec 10, 2025 at 12:01 AM <frank.chang@sifive.com> wrote:

> From: Frank Chang <frank.chang@sifive.com>
>
> This patch series fixes the spurious IRQ issue in the SiFive UART on Linux
> introduced after commit [1], which changed character transmission from
> synchronous to asynchronous.
>
> Currently, the txwm interrupt pending status is only updated when the
> asynchronous transmit handler runs. This can cause the txwm interrupt
> state to become unsynchronized between the SiFive UART and the interrupt
> controller.
>
> This issue has been observed on resource-constrained systems, where
> Linux reports spurious IRQ errors. In these cases, the asynchronous
> transmit handler is unable to drain the TX FIFO quickly enough to update
> the txwm pending status before software reads the IP register, which
> derives the txwm pending state directly from the actual number of
> characters in the TX FIFO.
>
> The remaining patches contain miscellaneous updates, including
> implementing txctrl.txen and rxctrl.rxen, update IRQ when rxctrl is
> written and remove the unused ip variable.
>
> [1] 53c1557b230986ab6320a58e1b2c26216ecd86d5
>
> Frank Chang (4):
>   hw/char: sifive_uart: Implement txctrl.txen and rxctrl.rxen
>   hw/char: sifive_uart: Sync txwm interrupt pending status after TX FIFO
>     enqueue
>   hw/char: sifive_uart: Update IRQ when rxctrl is written
>   hw/char: sifive_uart: Remove ip variable
>
>  hw/char/sifive_uart.c         | 46 ++++++++++++++++++++++++++++-------
>  include/hw/char/sifive_uart.h |  3 ++-
>  2 files changed, 39 insertions(+), 10 deletions(-)
>
> --
> 2.43.0
>
>