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

[Terin Stock]

Thanks Alan and Doug for your feedback. They've both been extremely helpful
in understanding what you're looking for in messages. A few responses below.

> I think your commit will be more compelling with additional data.  As
> Allen says it looks like you're not actually changing the delay.  You
> could include things like:
>
> * On systems with 100 HZ in the ideal case we'd end up delaying for 10
> ms - 20 ms when we used jiffies.  Now we'll get much closer to 1 ms.
>
> * Timer functions are not very high priority, so if we were running at
> a high load then we'd sometimes see much longer delays.  (NOTE: if you
> say this then please back it up with data--I think I've heard
> anecdotally that the normal timer functions can be quite delayed but I
> haven't done the research to back it up).  Presumably you could use
> ktime to measure delays before and after your patch and you could
> include this in the commit message.

Yes, this change decreases the delay, from a variable 10-20ms to 1ms.
I'll be sure to state this more clearly in the next version. I read Alan's
feedback as generally asking for the question. I'll run some tests with
ktime between now and then.

> It would also be good to document what device you were plugging in
> that you were having problems with and what system you were running
> on.  That would help someone else if they ever wanted to modify the
> same area of code and re-test.  They'd have a better chance of not
> re-breaking you.

This is a rather generic USB floppy drive. Is there a preference of
vid and pid and/or product and manufacturer strings?

> Overall nit: please CC LKML on patches.  If nothing else that makes
> them easier to find in lore.kernel.org/patchwork

To be clear, is this CC on the email envelope?

>> diff --git a/drivers/usb/dwc2/hcd_queue.c b/drivers/usb/dwc2/hcd_queue.c
>> index 301ced1618f8..2d0cfd7f2cfe 100644
>> --- a/drivers/usb/dwc2/hcd_queue.c
>> +++ b/drivers/usb/dwc2/hcd_queue.c
>> @@ -59,7 +59,7 @@
>>  #define DWC2_UNRESERVE_DELAY (msecs_to_jiffies(5))
>>
>>  /* If we get a NAK, wait this long before retrying */
>> -#define DWC2_RETRY_WAIT_DELAY (msecs_to_jiffies(1))
>> +#define DWC2_RETRY_WAIT_DELAY 1*1E6L
>>
>>  /**
>>   * dwc2_periodic_channel_available() - Checks that a channel is available 
>> for a
>> @@ -1465,9 +1465,9 @@ static void dwc2_deschedule_periodic(struct dwc2_hsotg 
>> *hsotg,
>>   *
>>   * @t: Pointer to wait_timer in a qh.
>>   */
>> -static void dwc2_wait_timer_fn(struct timer_list *t)
>> +static enum hrtimer_restart dwc2_wait_timer_fn(struct hrtimer *t)
>
> nit: please update function docstring to include a "Return:" clause now.

Sure, I'll fix this in the next version.

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

[Terin Stock]

Upon upgrading a Raspberry Pi 3B-based project from vanilla 4.14,
attempts to mount a floppy disk in a generic USB floppy drive would hang
until the floppy drive was removed from the system.

Tracing shows that during mounting the drive produces a large amount of
NAKed transactions, but would eventually continue. A previous commit
added a retry delay on NAKed transactions, using jiffies, that results
in indefinite NAKs in this scenario.

Modify the wait delay utilize the high resolution timer API to allow for
more accurately scheduled callbacks.

Fixes: 38d2b5fb75c1 ("usb: dwc2: host: Don't retry NAKed transactions right 
away")
Signed-off-by: Terin Stock 
---
 drivers/usb/dwc2/hcd.h   |2 +-
 drivers/usb/dwc2/hcd_queue.c |   17 ++---
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/drivers/usb/dwc2/hcd.h b/drivers/usb/dwc2/hcd.h
index 5502a501f516..93483dc37801 100644
--- a/drivers/usb/dwc2/hcd.h
+++ b/drivers/usb/dwc2/hcd.h
@@ -366,7 +366,7 @@ struct dwc2_qh {
u32 desc_list_sz;
u32 *n_bytes;
struct timer_list unreserve_timer;
-   struct timer_list wait_timer;
+   struct hrtimer wait_timer;
struct dwc2_tt *dwc_tt;
int ttport;
unsigned tt_buffer_dirty:1;
diff --git a/drivers/usb/dwc2/hcd_queue.c b/drivers/usb/dwc2/hcd_queue.c
index 301ced1618f8..2d0cfd7f2cfe 100644
--- a/drivers/usb/dwc2/hcd_queue.c
+++ b/drivers/usb/dwc2/hcd_queue.c
@@ -59,7 +59,7 @@
 #define DWC2_UNRESERVE_DELAY (msecs_to_jiffies(5))
 
 /* If we get a NAK, wait this long before retrying */
-#define DWC2_RETRY_WAIT_DELAY (msecs_to_jiffies(1))
+#define DWC2_RETRY_WAIT_DELAY 1*1E6L
 
 /**
  * dwc2_periodic_channel_available() - Checks that a channel is available for a
@@ -1465,9 +1465,9 @@ static void dwc2_deschedule_periodic(struct dwc2_hsotg 
*hsotg,
  *
  * @t: Pointer to wait_timer in a qh.
  */
-static void dwc2_wait_timer_fn(struct timer_list *t)
+static enum hrtimer_restart dwc2_wait_timer_fn(struct hrtimer *t)
 {
-   struct dwc2_qh *qh = from_timer(qh, t, wait_timer);
+   struct dwc2_qh *qh = container_of(t, struct dwc2_qh, wait_timer);
struct dwc2_hsotg *hsotg = qh->hsotg;
unsigned long flags;
 
@@ -1491,6 +1491,7 @@ static void dwc2_wait_timer_fn(struct timer_list *t)
}
 
spin_unlock_irqrestore(>lock, flags);
+   return HRTIMER_NORESTART;
 }
 
 /**
@@ -1521,7 +1522,8 @@ static void dwc2_qh_init(struct dwc2_hsotg *hsotg, struct 
dwc2_qh *qh,
/* Initialize QH */
qh->hsotg = hsotg;
timer_setup(>unreserve_timer, dwc2_unreserve_timer_fn, 0);
-   timer_setup(>wait_timer, dwc2_wait_timer_fn, 0);
+   hrtimer_init(>wait_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+   qh->wait_timer.function = _wait_timer_fn;
qh->ep_type = ep_type;
qh->ep_is_in = ep_is_in;
 
@@ -1690,7 +1692,7 @@ void dwc2_hcd_qh_free(struct dwc2_hsotg *hsotg, struct 
dwc2_qh *qh)
 * won't do anything anyway, but we want it to finish before we free
 * memory.
 */
-   del_timer_sync(>wait_timer);
+   hrtimer_cancel(>wait_timer);
 
dwc2_host_put_tt_info(hsotg, qh->dwc_tt);
 
@@ -1716,6 +1718,7 @@ int dwc2_hcd_qh_add(struct dwc2_hsotg *hsotg, struct 
dwc2_qh *qh)
 {
int status;
u32 intr_mask;
+   ktime_t delay;
 
if (dbg_qh(qh))
dev_vdbg(hsotg->dev, "%s()\n", __func__);
@@ -1734,8 +1737,8 @@ int dwc2_hcd_qh_add(struct dwc2_hsotg *hsotg, struct 
dwc2_qh *qh)
list_add_tail(>qh_list_entry,
  >non_periodic_sched_waiting);
qh->wait_timer_cancel = false;
-   mod_timer(>wait_timer,
- jiffies + DWC2_RETRY_WAIT_DELAY + 1);
+   delay = ktime_set(0, DWC2_RETRY_WAIT_DELAY);
+   hrtimer_start(>wait_timer, delay, HRTIMER_MODE_REL);
} else {
list_add_tail(>qh_list_entry,
  >non_periodic_sched_inactive);