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SubjectRe: [PATCH v4] usb: dwc2: host: Don't retry NAKed transactions right away
Felipe,

On Tue, Dec 12, 2017 at 10:30 AM, Douglas Anderson
<dianders@chromium.org> wrote:
> 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>
> Reviewed-by: Julius Werner <jwerner@chromium.org>
> Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
> Acked-by: John Youn <johnyoun@synopsys.com>
> ---
>
> Changes in v4:
> - Removed Cc for stable as per Felipe's request in v3
> - Rebased and squashed the two patches since Kees' timer stuff landed
> - Add John Youn's Ack.
>
> Changes in v3:
> - Add tested-by for Stefan Wahren
> - Sent to Felipe Balbi as candiate to land this.
> - Add Cc for stable (it's always been broken so go as far is as easy)
>
> Changes in v2:
> - Address http://crosreview.com/737520 feedback
>
> drivers/usb/dwc2/core.h | 1 +
> drivers/usb/dwc2/hcd.c | 7 ++++
> drivers/usb/dwc2/hcd.h | 9 +++++
> drivers/usb/dwc2/hcd_intr.c | 20 +++++++++++
> drivers/usb/dwc2/hcd_queue.c | 81 +++++++++++++++++++++++++++++++++++++++++---
> 5 files changed, 114 insertions(+), 4 deletions(-)

I don't mean to be a pest, but I'm hoping that we can land this
somewhere (if nothing else in your /next tree) just so it doesn't miss
another release cycle. If you're not so keen on collecting dwc2 host
patches these days, I can also see if Greg KH is willing to take this
directly into his tree. Please let me know. Thanks for your time!
:)

-Doug

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