[PATCH v3 0/2] USB: serial: ftdi_sio: latency_timer reliability fixes

[Chinna Mopurigari Naveen Kumar Reddy <naveen.reddy@ftdichip.com>]

Two independent correctness fixes for ftdi_sio:

  1. Retry transient errors (-ETIMEDOUT / -EPIPE / -EPROTO) on
     chip-side control transfers so that a single transient USB hiccup
     under bus load does not fail an otherwise-healthy one-shot
     configuration (e.g. a latency_timer sysfs write) back to
     userspace as -EIO.

  2. Make an explicit sysfs write to latency_timer authoritative so
     that a userspace write is not silently clamped to 1 by the
     ASYNC_LOW_LATENCY tty flag left set by an earlier TIOCSSERIAL
     (setserial(8), libftdi, certain tcsetattr paths).

Changes since v2:
  - patch 2/2: lower the override notice from dev_info() to dev_dbg()
    so the driver stays quiet in normal operation (Greg Kroah-Hartman).

Changes since v1:
  - Dropped the urb_defer_timer_ns and low_latency_defer_ns
    module-parameter patches, which addressed DMA-channel starvation on
    a host controller that does not enforce DMA-channel fairness.  As
    Greg Kroah-Hartman noted, that belongs in the host controller
    driver, not as a per-driver workaround in ftdi_sio.
  - Dropped the per-port low_latency sysfs attribute and its
    serialisation fix, which depended on the hrtimer infrastructure
    introduced by the dropped patch and have no standalone purpose
    without it.

Earlier postings:
  [v1] https://lore.kernel.org/linux-usb/cover.1781744946.git.naveen.reddy@ftdichip.com/
  [v2] https://lore.kernel.org/linux-usb/cover.1782121274.git.naveen.reddy@ftdichip.com/

Chinna Mopurigari Naveen Kumar Reddy (2):
  USB: serial: ftdi_sio: retry transient errors on chip-side control
    transfers
  USB: serial: ftdi_sio: make explicit latency_timer sysfs write
    authoritative

 drivers/usb/serial/ftdi_sio.c | 65 +++++++++++++++++++++++++++++++----
 1 file changed, 58 insertions(+), 7 deletions(-)


base-commit: ba3e43a9e601636f5edb54e259a74f96ca3b8fd8
-- 
2.43.0