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

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

Hello Roger, Peter, Pawel, Greg, Mathias,

On Tue Dec 17, 2024 at 10:00 PM CET, Roger Quadros wrote:
>
>
> On 13/12/2024 18:03, Théo Lebrun wrote:
> > 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.
>
> How is it a lie?
> The XHCI controller did loose its save/restore state during a PM operation.
> As far as XHCI is concerned this is an error. no?

The `xhci->quirks & XHCI_RESET_ON_RESUME` is exactly the same thing;
both the quirk and the flag we add have for purpose:

1. skipping this block

	if (!reinit_xhc) {
		retval = xhci_handshake(&xhci->op_regs->status,
					STS_CNR, 0, 10 * 1000 * 1000);
		// ...
		xhci_restore_registers(xhci);
		xhci_set_cmd_ring_deq(xhci);
		command = readl(&xhci->op_regs->command);
		command |= CMD_CRS;
		writel(command, &xhci->op_regs->command);
		if (xhci_handshake(&xhci->op_regs->status,
			      STS_RESTORE, 0, 100 * 1000)) {
			// ...
		}
	}

2. avoiding this warning:

	xhci_warn(xhci, "xHC error in resume, USBSTS 0x%x, Reinit\n", temp);

I don't think the block skipped is important in resume duration (to be
confirmed). But the xhci_warn() is not desired: we do not want to log
warnings if we know it is expected.

I'll think some more about it.

Thanks,

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