Re: [PATCH v6 4/5] xhci: introduce xhci->lost_power flag

[Roger Quadros <rogerq@kernel.org>]

On 10/12/2024 19:13, Théo Lebrun wrote:
> The XHCI_RESET_ON_RESUME quirk allows wrappers to signal that they
> expect a reset after resume. It is also used by some to enforce a XHCI
> reset on resume (see needs-reset-on-resume DT prop).
> 
> Some wrappers are unsure beforehands if they will reset. Add a mechanism
> to signal *at resume* if power has been lost. Parent devices can set
> this flag, that defaults to false.
> 
> The XHCI_RESET_ON_RESUME quirk still triggers a runtime_pm_get() on the
> controller. This is required as we do not know if a suspend will
> trigger a reset, so the best guess is to avoid runtime PM.
> 
> Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
> ---
>  drivers/usb/host/xhci.c | 3 ++-
>  drivers/usb/host/xhci.h | 6 ++++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
> index 5ebde8cae4fc44cdb997b0f61314e309bda56c0d..ae2c8daa206a87da24d58a62b0a0485ebf68cdd6 100644
> --- a/drivers/usb/host/xhci.c
> +++ b/drivers/usb/host/xhci.c
> @@ -1017,7 +1017,8 @@ int xhci_resume(struct xhci_hcd *xhci, pm_message_t msg)
>  
>  	spin_lock_irq(&xhci->lock);
>  
> -	if (hibernated || xhci->quirks & XHCI_RESET_ON_RESUME || xhci->broken_suspend)
> +	if (hibernated || xhci->quirks & XHCI_RESET_ON_RESUME ||
> +	    xhci->broken_suspend || xhci->lost_power)
>  		reinit_xhc = true;
>  
>  	if (!reinit_xhc) {
> diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
> index 4914f0a10cff42dbc1448dcf7908534d582c848e..32526df75925989d40cbe7d59a187c945f498a30 100644
> --- a/drivers/usb/host/xhci.h
> +++ b/drivers/usb/host/xhci.h
> @@ -1645,6 +1645,12 @@ struct xhci_hcd {
>  	unsigned		broken_suspend:1;
>  	/* Indicates that omitting hcd is supported if root hub has no ports */
>  	unsigned		allow_single_roothub:1;
> +	/*
> +	 * Signal from upper stacks that we lost power during system-wide
> +	 * suspend. Its default value is based on XHCI_RESET_ON_RESUME, meaning
> +	 * it is safe for wrappers to not modify lost_power at resume.
> +	 */
> +	unsigned                lost_power:1;

I suppose this is private to XHCI driver and not legitimate to be accessed
by another driver after HCD is instantiated?
Doesn't access to xhci_hcd need to be serialized via xhci->lock?

Just curious, what happens if you don't include patch 4 and 5?
Is USB functionality still broken for you?

Doesn't XHCI driver detect that power was lost and issue a reset in any case
via the below code

        /* re-initialize the HC on Restore Error, or Host Controller Error */
        if ((temp & (STS_SRE | STS_HCE)) &&
            !(xhci->xhc_state & XHCI_STATE_REMOVING)) {
                reinit_xhc = true;
                if (!xhci->broken_suspend)
                        xhci_warn(xhci, "xHC error in resume, USBSTS 0x%x, Reinit\n", temp);
        }

>  	/* cached extended protocol port capabilities */
>  	struct xhci_port_cap	*port_caps;
>  	unsigned int		num_port_caps;
> 

-- 
cheers,
-roger