linux-kernel - Re: [PATCH v2] xhci: fix runtime PM imbalance in USB2 resume

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Date:   Mon, 28 Feb 2022 12:56:39 +0200
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Henry Lin <henryl@...dia.com>, gregkh@...uxfoundation.org
Cc:     Mathias Nyman <mathias.nyman@...el.com>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] xhci: fix runtime PM imbalance in USB2 resume

On 25.2.2022 9.15, Henry Lin wrote:
> USB2 resume starts with usb_hcd_start_port_resume() in port status
> change handling for RESUME link state. usb_hcd_end_port_resume() call is
> needed to keep runtime PM balance.

For normal usb2 port resume the usb_hcd_end_port_resume() is called when resume
has been signaled for long enough in xhci_handle_usb2_port_link_resume().

This is also where driver directs the port to go from Resume state to U0.
Port can't do this without driver directing it.

If there's a failure during resume signaling (disconnect, reset, error) then
stale resume variables are detected in xhci_get_port_status() and 
usb_hcd_end_port_resume() is called.

I do now see a231ec41e6f6 ("xhci: refactor U0 link state handling in get_port_status") 
does change order of checking and clearing stale resume variables, but this should
only happen if the first port state we read is a fully enabled functional U0 state after
a failed resume.

Could you expand a bit how this was detected? 

> 
> Fixes: a231ec41e6f6 ("xhci: refactor U0 link state handling in get_port_status")
> Signed-off-by: Henry Lin <henryl@...dia.com>
> ---
>  drivers/usb/host/xhci-hub.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/usb/host/xhci-hub.c b/drivers/usb/host/xhci-hub.c
> index df3522dab31b..4a8b07b8ee01 100644
> --- a/drivers/usb/host/xhci-hub.c
> +++ b/drivers/usb/host/xhci-hub.c
> @@ -1090,6 +1090,8 @@ static void xhci_get_usb2_port_status(struct xhci_port *port, u32 *status,
>  		if (link_state == XDEV_U0) {
>  			bus_state->resume_done[portnum] = 0;
>  			clear_bit(portnum, &bus_state->resuming_ports);
> +			usb_hcd_end_port_resume(&port->rhub->hcd->self,
> +						portnum);

This will call usb_hcd_end_port_resume() every time port is in normal enabled U0 state
even if resume was never signaled, or port suspended.

-Mathias