Our Baikal-T1 SoC is equipped with DWC USB3 IP core as a USB2.0 bus controller. In general the DWC USB3 driver is working well for it except the ULPI-bus part. We've found out that the DWC USB3 ULPI-bus driver detected PHY with VID:PID tuple as 0x0000:0x0000, which of course wasn't true since it was supposed to be 0x0424:0x0006. After a short digging inside the ulpi.c code and studying the DWC USB3 documentation, it has been discovered that the ULPI bus IO ops didn't work quite correct. The busy-loop had stopped waiting before the actual operation was finished. We found out that the problem was caused by several bugs hidden in the DWC USB3 ULPI-bus IO implementation.
First of all in accordance with the DWC USB3 databook [1] the ULPI IO busy-loop is supposed to use the GUSB2PHYACCn.VStsDone flag as an indication of the PHY vendor control access completion. Instead it polled the GUSB2PHYACCn.VStsBsy flag, which as we discovered can be cleared a bit before the VStsDone flag. Secondly having the simple counter-based loop in the modern kernel is really a weak design of the busy-looping pattern especially seeing the ULPI operations delay can be easily estimated [2], since the bus clock is fixed to 60MHz. Finally the root cause of the denoted in the prologue problem was due to the Suspend PHY DWC USB3 feature perception. The commit e0082698b689 ("usb: dwc3: ulpi: conditionally resume ULPI PHY") introduced the Suspend USB2.0 HS/FS/LS PHY regression as the Low-power consumption mode would be disable after a first attempt to read/write from the ULPI PHY control registers, and still didn't fix the problem it was originally intended for since the very first attempt of the ULPI PHY control registers IO would need much more time than the busy-loop provided. So instead of disabling the Suspend USB2.0 HS/FS/LS PHY feature we suggest to just extend the busy-loop delay in case if the GUSB2PHYCFGn.SusPHY flag set to 1. By doing so we'll eliminate the regression and the fix the false busy-loop timeout problem. [1] Synopsys DesignWare Cores SuperSpeed USB 3.0 xHCI Host Controller Databook, 2.70a, December 2013, p.388 [1] UTMI+ Low Pin Interface (ULPI) Specification, Revision 1.1, October 20, 2004, pp. 30 - 36. Fixes: e0082698b689 ("usb: dwc3: ulpi: conditionally resume ULPI PHY") Fixes: 88bc9d194ff6 ("usb: dwc3: add ULPI interface support") Signed-off-by: Serge Semin <sergey.se...@baikalelectronics.ru> Cc: Alexey Malahov <alexey.mala...@baikalelectronics.ru> Cc: Pavel Parkhomenko <pavel.parkhome...@baikalelectronics.ru> Cc: linux-...@vger.kernel.org Cc: linux-kernel@vger.kernel.org Serge Semin (3): usb: dwc3: ulpi: Use VStsDone to detect PHY regs access completion usb: dwc3: ulpi: Replace CPU-based busyloop with Protocol-based one usb: dwc3: ulpi: Fix USB2.0 HS/FS/LS PHY suspend regression drivers/usb/dwc3/core.h | 1 + drivers/usb/dwc3/ulpi.c | 38 +++++++++++++++++++++----------------- 2 files changed, 22 insertions(+), 17 deletions(-) -- 2.27.0