Re: [PATCH v3 1/2] usb: dwc2: host: Don't retry NAKed transactions right away

[Felipe Balbi <balbi@kernel.org>]

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Hi,

Douglas Anderson <dianders@chromium.org> writes:
> On rk3288-veyron devices on Chrome OS it was found that plugging in an
> Arduino-based USB device could cause the system to lockup, especially
> if the CPU Frequency was at one of the slower operating points (like
> 100 MHz / 200 MHz).
>
> Upon tracing, I found that the following was happening:
> * The USB device (full speed) was connected to a high speed hub and
>   then to the rk3288.  Thus, we were dealing with split transactions,
>   which is all handled in software on dwc2.
> * Userspace was initiating a BULK IN transfer
> * When we sent the SSPLIT (to start the split transaction), we got an
>   ACK.  Good.  Then we issued the CSPLIT.
> * When we sent the CSPLIT, we got back a NAK.  We immediately (from
>   the interrupt handler) started to retry and sent another SSPLIT.
> * The device kept NAKing our CSPLIT, so we kept ping-ponging between
>   sending a SSPLIT and a CSPLIT, each time sending from the interrupt
>   handler.
> * The handling of the interrupts was (because of the low CPU speed and
>   the inefficiency of the dwc2 interrupt handler) was actually taking
>   _longer_ than it took the other side to send the ACK/NAK.  Thus we
>   were _always_ in the USB interrupt routine.
> * The fact that USB interrupts were always going off was preventing
>   other things from happening in the system.  This included preventing
>   the system from being able to transition to a higher CPU frequency.
>
> As I understand it, there is no requirement to retry super quickly
> after a NAK, we just have to retry sometime in the future.  Thus one
> solution to the above is to just add a delay between getting a NAK and
> retrying the transmission.  If this delay is sufficiently long to get
> out of the interrupt routine then the rest of the system will be able
> to make forward progress.  Even a 25 us delay would probably be
> enough, but we'll be extra conservative and try to delay 1 ms (the
> exact amount depends on HZ and the accuracy of the jiffy and how close
> the current jiffy is to ticking, but could be as much as 20 ms or as
> little as 1 ms).
>
> Presumably adding a delay like this could impact the USB throughput,
> so we only add the delay with repeated NAKs.
>
> NOTE: Upon further testing of a pl2303 serial adapter, I found that
> this fix may help with problems there.  Specifically I found that the
> pl2303 serial adapters tend to respond with a NAK when they have
> nothing to say and thus we end with this same sequence.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Cc: stable@vger.kernel.org
> Reviewed-by: Julius Werner <jwerner@chromium.org>
> Tested-by: Stefan Wahren <stefan.wahren@i2se.com>

This seems too big for -rc or -stable inclusion. In any case, this
doesn't apply to my testing/next branch. Care to rebase and collect acks
you received while doing that?

-- 
balbi

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