Re: [PATCH v3 3/4] PCI: Create new reset method to force SBR for CXL

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

> >   
> >> +
> >> +	bridge = pci_upstream_bridge(dev);
> >> +	if (!bridge)
> >> +		return -ENOTTY;
> >> +
> >> +	dvsec = cxl_port_dvsec(bridge);
> >> +	if (!dvsec)
> >> +		return -ENOTTY;
> >> +
> >> +	if (probe)
> >> +		return 0;
> >> +
> >> +	pci_cfg_access_lock(bridge);
> >> +	rc = pci_read_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL, &reg);
> >> +	if (rc) {
> >> +		rc = -ENOTTY;
> >> +		goto out;
> >> +	}
> >> +
> >> +	if (!(reg & PCI_DVSEC_CXL_PORT_CTL_UNMASK_SBR)) {
> >> +		val = reg | PCI_DVSEC_CXL_PORT_CTL_UNMASK_SBR;
> >> +		pci_write_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL,
> >> +				      val);
> >> +	} else {
> >> +		val = reg;
> >> +	}
> >> +
> >> +	rc = pci_reset_bus_function(dev, probe);
> >> +
> >> +	if (reg != val)
> >> +		pci_write_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL, reg);
> >> +
> >> +out:
> >> +	pci_cfg_access_unlock(bridge);  
> > 
> > Maybe a guard() use case to allow early returns in error paths?  
> 
> I'm not seeing a good way to do it. pci_cfg_access_lock/unlock() isn't like your typical lock/unlock. It locks, changes some pci_dev internal stuff, and then unlocks in both functions. The pci_lock isn't being held after lock() call.
> 

You've lost me.

Why does guard() care about the internals?

All it does is stash a copy of the '_lock' - here the bridge struct pci_dev then call the _unlock
on it when the stashed copy of that pointer when it goes out of scope.

Those functions don't need to hold a conventional lock.  Though in this case
I believe the lock is effectively pci_dev->block_cfg_access.

FWIW we do the similar in IIO (with a conditional lock for extra fun :)
https://elixir.bootlin.com/linux/v6.9-rc2/source/include/linux/iio/iio.h#L650
That is setting a flag much like this one.  Don't look too closely at that though
as it evolved into a slightly odd form and needs a revisit.

This was a possible nice to have, not something I care that much about
in this patch set so feel free to not do it :)

Jonathan



> >   
> >> +	return rc;
> >> +}
> >> +
> >>  void pci_dev_lock(struct pci_dev *dev)
> >>  {
> >>  	/* block PM suspend, driver probe, etc. */
> >> @@ -5066,6 +5109,7 @@ static const struct pci_reset_fn_method pci_reset_fn_methods[] = {
> >>  	{ pci_af_flr, .name = "af_flr" },
> >>  	{ pci_pm_reset, .name = "pm" },
> >>  	{ pci_reset_bus_function, .name = "bus" },
> >> +	{ cxl_reset_bus_function, .name = "cxl_bus" },
> >>  };
> >>  
> >>  static ssize_t reset_method_show(struct device *dev,
> >> diff --git a/include/linux/pci.h b/include/linux/pci.h
> >> index 16493426a04f..235f37715a43 100644
> >> --- a/include/linux/pci.h
> >> +++ b/include/linux/pci.h
> >> @@ -51,7 +51,7 @@
> >>  			       PCI_STATUS_PARITY)
> >>  
> >>  /* Number of reset methods used in pci_reset_fn_methods array in pci.c */
> >> -#define PCI_NUM_RESET_METHODS 7
> >> +#define PCI_NUM_RESET_METHODS 8
> >>  
> >>  #define PCI_RESET_PROBE		true
> >>  #define PCI_RESET_DO_RESET	false  
> >   
>