Re: [PATCH v3 2/2] USB: serial: ftdi_sio: make explicit latency_timer sysfs write authoritative

[Johan Hovold <johan@kernel.org>]

On Tue, Jun 23, 2026 at 04:03:21PM +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.  The override is logged at debug level
> (dev_dbg) for diagnostics only.
> 
> 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.

No, I don't think we should be changing this now.

The driver has behaved this way since 2009 and commit 557aaa7ffab6
("ft232: support the ASYNC_LOW_LATENCY flag") which explicitly let the
standard setserial interface override the "obscure chip specific"
interface.

Also note that the value written to sysfs does take effect once the
low-latency flag is cleared so we can't really return an error like
-EBUSY either.

I don't know how libftdi uses the flag, but it generally would not even
be set unless you explicitly set it using setserial.

Perhaps you can send a fix to libftdi so that it uses the sysfs
interface directly and leaves the flag alone if that's the library
that's causing the flag to unknowingly be left set.

Johan