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

From: Mathias Nyman <mathias.nyman@linux.intel.com>
To: Henry Lin <henryl@nvidia.com>, gregkh@linuxfoundation.org
Cc: Mathias Nyman <mathias.nyman@intel.com>,
	linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] xhci: fix runtime PM imbalance in USB2 resume
Date: Mon, 28 Feb 2022 12:56:39 +0200	[thread overview]
Message-ID: <2ef7da52-d8ad-05ca-bcb6-06bd6bb6f9d3@linux.intel.com> (raw)
In-Reply-To: <20220225071506.22012-1-henryl@nvidia.com>

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@nvidia.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