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

["Théo Lebrun" <theo.lebrun@bootlin.com>]

On Thu Dec 12, 2024 at 1:37 PM CET, Roger Quadros wrote:
> 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?

Yes it is private.

> Doesn't access to xhci_hcd need to be serialized via xhci->lock?

Good question. In theory maybe. In practice I don't see how
cdns_host_resume(), called by cdns_resume(), could clash with anything
else. I'll add that to be safe.

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

No it works fine. Patches 4+5 are only there to avoid the below warning.
Logging "xHC error in resume" is a lie, so I want to avoid it.

> 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;
> > 

I'll wait for your opinion on the [PATCH v6 2/5] email thread before
sending a new revision.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com