Re: [PATCH v2 3/4] PCI: Add function for parsing 'slot-power-limit-milliwatt' DT property

["Pali Rohár" <pali@kernel.org>]

On Thursday 24 March 2022 12:13:37 Bjorn Helgaas wrote:
> On Wed, Mar 02, 2022 at 03:57:32PM +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>
> > ---
> > 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..549a404bd536 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;
> > +	} else if (slot_power_limit_mw <= 600*1000) {
> > +		value = 0xF0 + (slot_power_limit_mw / 1000 - 250) / 25;
> > +		scale = 0;
> > +	} else {
> > +		value = 0xFF;
> > +		scale = 0;
> 
> The purpose of this function is to return values that can be
> programmed into the Slot Capabilities register.  The 0xFF Slot Power
> Limit Value is reserved, and I don't think we should use it until the
> spec defines a meaning for it.
> 
> If the DT tells us 800W is available, we'll put 0xFF in Slot Power
> Limit Value.  If the spec eventually defines (0xFF, 0) to mean "1000W
> available", a device may try to consume all 1000W, which will not
> work.
> 
> If slot_power_limit_mw > 600*1000, I think we should advertise 600W
> available (value 0xFE, scale 0) and return 600W (600*1000).
> 
> Bjorn

Ok, I will send a v3 with this change.