Re: [PATCH v2] usb: dwc2: host: use hrtimer for NAK retries

[Minas Harutyunyan]

On 11/14/2018 3:30 AM, Doug Anderson wrote:
> Hi,
> 
> On Sun, Sep 9, 2018 at 9:24 PM Terin Stock <terin@xxxxxxxxxxxxxx> wrote:
>>
>> 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: 38d2b5fb75c1 ("usb: dwc2: host: Don't retry NAKed transactions right away")
>> Signed-off-by: Terin Stock <terin@xxxxxxxxxxxxxx>
>> ---
>>   drivers/usb/dwc2/hcd.h       |    2 +-
>>   drivers/usb/dwc2/hcd_queue.c |   19 ++++++++++++-------
>>   2 files changed, 13 insertions(+), 8 deletions(-)
> 
> Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> 
Acked-by: Minas Harutyunyan <hminas@xxxxxxxxxxxx>