Hi,

On Tue, Dec 12, 2017 at 3:06 AM, Felipe Balbi balbi@kernel.org wrote:

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.

I've removed the stable tag at your request. I originally added it at your request in response to v2 of this patch. I'd agree that it's a pretty big patch and therefore "risky" to pick back to stable. ...but it does fix a bug reported by several people on the mailing lists, so I'll leave it to your discretion. Previously in relation to the stable tag, I had mentioned:

It's a little weird since it doesn't "fix" any specific commit, so I guess it will be up to stable folks to decide how far to go back. The dwc2 devices I work with are actually on 3.14, but we have some pretty massive backports related to dwc2 there...

In any case, this doesn't apply to my testing/next branch. Care to rebase and collect acks you received while doing that?

Sure, no problem. I've posted v4 with John Youn's Ack. The reason v3 didn't apply is that you've now got commit e99e88a9d2b0 ("treewide: setup_timer() -> timer_setup()"). Originally my plan was to beat that patch into the kernel and then I'd do the timer conversion myself. That was patch #2 in the v3 series, AKA https://patchwork.kernel.org/patch/10032935/. ...but since I failed to beat Kees' patch in, I've now squashed patches #1 and #2 together and resolved the trivial conflict.

If anyone were thinking of trying to backport this patch to older kernels (where they presumably don't have Kees's timer patch) they can always use the v3 patch posted here as a reference for how to make things work. ;)

-Doug