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

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Date:   Tue, 13 Nov 2018 15:30:23 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Terin Stock <terin@...instock.com>
Cc:     linux-usb@...r.kernel.org, Julius Werner <jwerner@...omium.org>,
        Dmitry Torokhov <dtor@...omium.org>,
        Minas Harutyunyan <hminas@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2] usb: dwc2: host: use hrtimer for NAK retries

Hi,

On Sun, Sep 9, 2018 at 9:24 PM Terin Stock <terin@...instock.com> 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@...instock.com>
> ---
>  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@...omium.org>
Cc: stable@...r.kernel.org