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38d2b5fb75
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> Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.