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

[Roger Quadros <rogerq@kernel.org>]

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?

> 
>> 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.

Sorry for the delay. I'm not an expert in PM but will give my opinion there anyways.

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

-- 
cheers,
-roger