Re: [PATCH v3 3/4] PCI: Add function for parsing 'slot-power-limit-milliwatt' DT property

[Pali Rohár]

On Friday 08 April 2022 10:27:50 Bjorn Helgaas wrote:
> On Fri, Mar 25, 2022 at 10:38:26AM +0100, Pali Rohár wrote:
> > Add function of_pci_get_slot_power_limit(), which parses the
> > 'slot-power-limit-milliwatt' DT property, returning the value in
> > milliwatts and in format ready for the PCIe Slot Capabilities Register.
> > 
> > Signed-off-by: Pali Rohár <pali@xxxxxxxxxx>
> > Signed-off-by: Marek Behún <kabel@xxxxxxxxxx>
> > Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
> > ---
> > Changes in v3:
> > * Set 600 W when DT slot-power-limit-milliwatt > 600 W
> > Changes in v2:
> > * Added support for PCIe 6.0 slot power limit encodings
> > * Round down slot power limit value
> > ---
> >  drivers/pci/of.c  | 64 +++++++++++++++++++++++++++++++++++++++++++++++
> >  drivers/pci/pci.h | 15 +++++++++++
> >  2 files changed, 79 insertions(+)
> > 
> > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > index cb2e8351c2cc..5ebff26edd41 100644
> > --- a/drivers/pci/of.c
> > +++ b/drivers/pci/of.c
> > @@ -633,3 +633,67 @@ int of_pci_get_max_link_speed(struct device_node *node)
> >  	return max_link_speed;
> >  }
> >  EXPORT_SYMBOL_GPL(of_pci_get_max_link_speed);
> > +
> > +/**
> > + * of_pci_get_slot_power_limit - Parses the "slot-power-limit-milliwatt"
> > + *				 property.
> > + *
> > + * @node: device tree node with the slot power limit information
> > + * @slot_power_limit_value: pointer where the value should be stored in PCIe
> > + *			    Slot Capabilities Register format
> > + * @slot_power_limit_scale: pointer where the scale should be stored in PCIe
> > + *			    Slot Capabilities Register format
> > + *
> > + * Returns the slot power limit in milliwatts and if @slot_power_limit_value
> > + * and @slot_power_limit_scale pointers are non-NULL, fills in the value and
> > + * scale in format used by PCIe Slot Capabilities Register.
> > + *
> > + * If the property is not found or is invalid, returns 0.
> > + */
> > +u32 of_pci_get_slot_power_limit(struct device_node *node,
> > +				u8 *slot_power_limit_value,
> > +				u8 *slot_power_limit_scale)
> > +{
> > +	u32 slot_power_limit_mw;
> > +	u8 value, scale;
> > +
> > +	if (of_property_read_u32(node, "slot-power-limit-milliwatt",
> > +				 &slot_power_limit_mw))
> > +		slot_power_limit_mw = 0;
> > +
> > +	/* Calculate Slot Power Limit Value and Slot Power Limit Scale */
> > +	if (slot_power_limit_mw == 0) {
> > +		value = 0x00;
> > +		scale = 0;
> > +	} else if (slot_power_limit_mw <= 255) {
> > +		value = slot_power_limit_mw;
> > +		scale = 3;
> > +	} else if (slot_power_limit_mw <= 255*10) {
> > +		value = slot_power_limit_mw / 10;
> > +		scale = 2;
> > +	} else if (slot_power_limit_mw <= 255*100) {
> > +		value = slot_power_limit_mw / 100;
> > +		scale = 1;
> > +	} else if (slot_power_limit_mw <= 239*1000) {
> > +		value = slot_power_limit_mw / 1000;
> > +		scale = 0;
> > +	} else if (slot_power_limit_mw <= 250*1000) {
> > +		value = 0xF0;
> > +		scale = 0;
> 
> I think the spec is poorly worded here.  PCIe r6.0, sec 7.5.3.9, says:
> 
>   F0h   > 239 W and <= 250 W Slot Power Limit
> 
> I don't think it's meaningful for the spec to include a range here.
> The amount of power the slot can supply has a single maximum.  I
> suspect the *intent* of F0h/00b is that a device in the slot may
> consume up to 250W.
> 
> Your code above would mean that slot_power_limit_mw == 245,000 would
> cause the slot to advertise F0h/00b (250W), which seems wrong.

So for slot_power_limit_mw == 245 W we should set following values?

  slot_power_limit_mw = 239 W
  value = 0xF0
  scale = 0

> I think we should do something like this instead:
> 
>   scale = 0;
>   if (slot_power_limit_mw >= 600*1000) {
>     value = 0xFE;
>     slot_power_limit_mw = 600*1000;
>   } else if (slot_power_limit_mw >= 575*1000) {
>     value = 0xFD;
>     slot_power_limit_mw = 575*1000;
>   } ...

This is already implemented in branch:

  } else if (slot_power_limit_mw <= 600*1000) {
  	value = 0xF0 + (slot_power_limit_mw / 1000 - 250) / 25;
  	scale = 0;

I will just add reducing of final slot_power_limit_mw value.

> I raised an issue with the PCI SIG about this.
> 
> > +	} else if (slot_power_limit_mw <= 600*1000) {
> > +		value = 0xF0 + (slot_power_limit_mw / 1000 - 250) / 25;
> > +		scale = 0;
> > +	} else {
> > +		value = 0xFE;
> > +		scale = 0;
> > +	}
> > +
> > +	if (slot_power_limit_value)
> > +		*slot_power_limit_value = value;
> > +
> > +	if (slot_power_limit_scale)
> > +		*slot_power_limit_scale = scale;
> > +
> > +	return slot_power_limit_mw;
> 
> If the DT tells us 800W is available, we'll store (FEh/00b), which
> means the slot can advertise to a downstream device that 600W is
> available.  I think that's correct, since the current spec doesn't
> provide a way to encode any value larger than 600W.
> 
> But the function still returns 800,000 mW, which means the next patch will
> print:
> 
>   %s: Slot power limit 800.0W
> 
> even though it programs Slot Capabilities to advertise 600W.
> That's why I suggested setting slot_power_limit_mw = 600*1000 above.

Ok, I will update slot_power_limit_mw value in next patch version.

> > +}
> > +EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
> > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > index 3d60cabde1a1..e10cdec6c56e 100644
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -627,6 +627,9 @@ struct device_node;
> >  int of_pci_parse_bus_range(struct device_node *node, struct resource *res);
> >  int of_get_pci_domain_nr(struct device_node *node);
> >  int of_pci_get_max_link_speed(struct device_node *node);
> > +u32 of_pci_get_slot_power_limit(struct device_node *node,
> > +				u8 *slot_power_limit_value,
> > +				u8 *slot_power_limit_scale);
> >  void pci_set_of_node(struct pci_dev *dev);
> >  void pci_release_of_node(struct pci_dev *dev);
> >  void pci_set_bus_of_node(struct pci_bus *bus);
> > @@ -653,6 +656,18 @@ of_pci_get_max_link_speed(struct device_node *node)
> >  	return -EINVAL;
> >  }
> >  
> > +static inline u32
> > +of_pci_get_slot_power_limit(struct device_node *node,
> > +			    u8 *slot_power_limit_value,
> > +			    u8 *slot_power_limit_scale)
> > +{
> > +	if (slot_power_limit_value)
> > +		*slot_power_limit_value = 0;
> > +	if (slot_power_limit_scale)
> > +		*slot_power_limit_scale = 0;
> > +	return 0;
> > +}
> > +
> >  static inline void pci_set_of_node(struct pci_dev *dev) { }
> >  static inline void pci_release_of_node(struct pci_dev *dev) { }
> >  static inline void pci_set_bus_of_node(struct pci_bus *bus) { }
> > -- 
> > 2.20.1
> > 
> > 
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