Re: [PATCH v1] Revert "usb: xhci: Implement xhci_handshake_check_state() helper"
From: Mathias Nyman
Date: Mon May 19 2025 - 08:53:37 EST
On 17.5.2025 7.39, Roy Luo wrote:
This reverts commit 6ccb83d6c4972ebe6ae49de5eba051de3638362c.
Commit 6ccb83d6c497 ("usb: xhci: Implement xhci_handshake_check_state()
helper") was introduced to workaround watchdog timeout issues on some
platforms, allowing xhci_reset() to bail out early without waiting
for the reset to complete.
Skipping the xhci handshake during a reset is a dangerous move. The
xhci specification explicitly states that certain registers cannot
be accessed during reset in section 5.4.1 USB Command Register (USBCMD),
Host Controller Reset (HCRST) field:
"This bit is cleared to '0' by the Host Controller when the reset
process is complete. Software cannot terminate the reset process
early by writinga '0' to this bit and shall not write any xHC
Operational or Runtime registers until while HCRST is '1'."
This behavior causes a regression on SNPS DWC3 USB controller with
dual-role capability. When the DWC3 controller exits host mode and
removes xhci while a reset is still in progress, and then tries to
configure its hardware for device mode, the ongoing reset leads to
register access issues; specifically, all register reads returns 0.
These issues extend beyond the xhci register space (which is expected
during a reset) and affect the entire DWC3 IP block, causing the DWC3
device mode to malfunction.
I agree with you and Thinh that waiting for the HCRST bit to clear during
reset is the right thing to do, especially now when we know skipping it
causes issues for SNPS DWC3, even if it's only during remove phase.
But reverting this patch will re-introduce the issue originally worked
around by Udipto Goswami, causing regression.
Best thing to do would be to wait for HCRST to clear for all other platforms
except the one with the issue.
Udipto Goswami, can you recall the platforms that needed this workaroud?
and do we have an easy way to detect those?
Thanks
Mathias