• Terin Stock's avatar
    usb: dwc2: host: use hrtimer for NAK retries · 6ed30a7d
    Terin Stock authored
    Modify the wait delay utilize the high resolution timer API to allow for
    more precisely scheduled callbacks.
    
    A previous commit added a 1ms retry delay after multiple consecutive
    NAKed transactions using jiffies. On systems with a low timer interrupt
    frequency, this delay may be significantly longer than specified,
    resulting in misbehavior with some USB devices.
    
    This scenario was reached on a Raspberry Pi 3B with a Macally FDD-USB
    floppy drive (identified as 0424:0fdc Standard Microsystems Corp.
    Floppy, based on the USB97CFDC USB FDC). With the relay delay, the drive
    would be unable to mount a disk, replying with NAKs until the device was
    reset.
    
    Using ktime, the delta between starting the timer (in dwc2_hcd_qh_add)
    and the callback function can be determined. With the original delay
    implementation, this value was consistently approximately 12ms. (output
    in us).
    
        <idle>-0     [000] ..s.  1600.559974: dwc2_wait_timer_fn: wait_timer delta: 11976
        <idle>-0     [000] ..s.  1600.571974: dwc2_wait_timer_fn: wait_timer delta: 11977
        <idle>-0     [000] ..s.  1600.583974: dwc2_wait_timer_fn: wait_timer delta: 11976
        <idle>-0     [000] ..s.  1600.595974: dwc2_wait_timer_fn: wait_timer delta: 11977
    
    After converting the relay delay to using a higher resolution timer, the
    delay was much closer to 1ms.
    
        <idle>-0     [000] d.h.  1956.553017: dwc2_wait_timer_fn: wait_timer delta: 1002
        <idle>-0     [000] d.h.  1956.554114: dwc2_wait_timer_fn: wait_timer delta: 1002
        <idle>-0     [000] d.h.  1957.542660: dwc2_wait_timer_fn: wait_timer delta: 1004
        <idle>-0     [000] d.h.  1957.543701: dwc2_wait_timer_fn: wait_timer delta: 1002
    
    The floppy drive operates properly with delays up to approximately 5ms,
    and sends NAKs for any delays that are longer.
    
    Fixes: 38d2b5fb ("usb: dwc2: host: Don't retry NAKed transactions right away")
    Cc: <stable@vger.kernel.org>
    Reviewed-by: default avatarDouglas Anderson <dianders@chromium.org>
    Acked-by: default avatarMinas Harutyunyan <hminas@synopsys.com>
    Signed-off-by: default avatarTerin Stock <terin@terinstock.com>
    Signed-off-by: default avatarFelipe Balbi <felipe.balbi@linux.intel.com>
    6ed30a7d
hcd.h 28.2 KB