Re: [PATCH 4/6] PCI: Add function for parsing 'slot-power-limit-milliwatt' DT property

["Pali Rohár" <pali@kernel.org>]

On Thursday 24 February 2022 14:47:15 Bjorn Helgaas wrote:
> On Tue, Feb 22, 2022 at 05:31:56PM +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@kernel.org>
> > Signed-off-by: Marek Behún <kabel@kernel.org>
> > Reviewed-by: Rob Herring <robh@kernel.org>
> > Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> > ---
> >  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..2b0c0a3641a8 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;
> 
> Including "mw" or similar reference to the units would give a hint of
> how to relate the code to the spec.
> 
> > +	u8 value, scale;
> > +
> > +	if (of_property_read_u32(node, "slot-power-limit-milliwatt",
> > +				 &slot_power_limit))
> > +		slot_power_limit = 0;
> > +
> > +	/* Calculate Slot Power Limit Value and Slot Power Limit Scale */
> 
> Add a spec reference to PCIe r6.0, sec 7.5.3.9.  IIUC, this supports
> up to 300W, which was what r5.0 defined, but r6.0 added values up to
> 0xfe (600W).

I did not know about it and I have not seen/read r6.0.

It would be nice if somebody with access to r6.0 send a patch to lspci
utility, so we could write support for 600W based on lspci parser.

> > +	if (slot_power_limit == 0) {
> > +		value = 0x00;
> > +		scale = 0;
> > +	} else if (slot_power_limit <= 255) {
> > +		value = slot_power_limit;
> > +		scale = 3;
> > +	} else if (slot_power_limit <= 255*10) {
> > +		value = slot_power_limit / 10;
> > +		scale = 2;
> > +	} else if (slot_power_limit <= 255*100) {
> > +		value = slot_power_limit / 100;
> > +		scale = 1;
> > +	} else if (slot_power_limit <= 239*1000) {
> > +		value = slot_power_limit / 1000;
> > +		scale = 0;
> > +	} else if (slot_power_limit <= 250*1000) {
> > +		value = 0xF0;
> > +		scale = 0;
> > +	} else if (slot_power_limit <= 275*1000) {
> > +		value = 0xF1;
> > +		scale = 0;
> > +	} else {
> > +		value = 0xF2;
> > +		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;
> 
> If "slot-power-limit-milliwatt" contains a value larger than can be
> represented in "value" and "scale", the return value will not agree
> with value/scale, will it?

In previous version 0xF2 was reserved for values above 275 W. So for me
it looked like a correct solution.

> Currently you only use the return value for a log message, so no real
> harm yet, other than the fact that we might print "Slot power limit
> 1000.0W" when the hardware will only advertise 600W available.
> 
> Also, if "slot-power-limit-milliwatt" contains something like
> 260000 mW (260 W), we'll return 0xF1/0, so the hardware will
> advertise 275 W available.

There is no way how to encode 260 W. It is possible only 250 W or 275 W,
and nothing between. I chose to round value to upper limit. What do you
prefer in these cases? Upper or lower limit?

> > +}
> > +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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