Re: [PATCH v2 2/2] USB: serial: ftdi_sio: make explicit latency_timer sysfs write authoritative

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Jun 22, 2026 at 05:43:40PM +0800, Chinna Mopurigari Naveen Kumar Reddy wrote:
> write_latency_timer() clamps the value programmed into the FT chip's
> per-channel latency_timer register to 1 whenever ASYNC_LOW_LATENCY is
> set in priv->flags.  ASYNC_LOW_LATENCY is set by userspace via
> TIOCSSERIAL, used by setserial(8), libftdi and certain tcsetattr
> paths.  The interaction with the existing sysfs latency_timer
> attribute is surprising: once any of those tools has set the flag, a
> later write of "16" (or any other value) to
> /sys/bus/usb-serial/devices/ttyUSBx/latency_timer is silently
> clamped to 1 and never reaches the chip.
> 
> The store path is the most explicit way userspace can ask for a
> particular latency_timer value; treat it as authoritative.  On an
> explicit sysfs write, clear ASYNC_LOW_LATENCY before calling
> write_latency_timer() so the requested value is what the chip
> register actually receives.  Emit a dev_info() so the override is
> visible.
> 
> Reads continue to honour ASYNC_LOW_LATENCY (returning "1") so any
> userspace that previously inspected the attribute to confirm
> low-latency mode keeps working until it does its own explicit write.
> 
> Signed-off-by: Chinna Mopurigari Naveen Kumar Reddy <naveen.reddy@ftdichip.com>
> ---
>  drivers/usb/serial/ftdi_sio.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
> index 7aaa7fc1be71..e7f13eca7ae6 100644
> --- a/drivers/usb/serial/ftdi_sio.c
> +++ b/drivers/usb/serial/ftdi_sio.c
> @@ -1683,6 +1683,23 @@ static ssize_t latency_timer_store(struct device *dev,
>  	if (kstrtou8(valbuf, 10, &v))
>  		return -EINVAL;
>  
> +	/*
> +	 * An explicit sysfs write wins over the legacy ASYNC_LOW_LATENCY
> +	 * tty-flag override.  Without this, if any userspace tool
> +	 * (setserial(8), libftdi, certain tcsetattr paths) had set
> +	 * ASYNC_LOW_LATENCY via TIOCSSERIAL, write_latency_timer() would
> +	 * silently clamp the chip register to 1 regardless of what was
> +	 * written to sysfs.  Clearing the flag here makes sysfs the
> +	 * authoritative source so the next chip-side write uses exactly
> +	 * the value the caller asked for.
> +	 */
> +	if (priv->flags & ASYNC_LOW_LATENCY) {
> +		dev_info(&port->dev,
> +			 "explicit latency_timer=%u clears ASYNC_LOW_LATENCY flag\n",
> +			 v);

When drivers work properly, they are quiet.  Make this a debugging
message at the most please.

thanks,

greg k-h