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

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Apr 11, 2022 at 01:14:07PM +0200, Pali Rohár wrote:
> 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@kernel.org>
> > > Signed-off-by: Marek Behún <kabel@kernel.org>
> > > Reviewed-by: Rob Herring <robh@kernel.org>
> > > ---
> > > 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 Slot Cap should never advertise more power than the slot can
supply.  So if the DT tells us the slot can supply 245 W, I don't
think Slot Cap should advertise that it can supply 250 W.  I think we
should drop down to the next lower possible value, which is 239 W
(value 0xEF, scale 0).  I think this is what your v4 does.

> > 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;

OK, I was thinking there was a hole here, but I guess not.  I think do
think it's easier to read and verify if it's structured as "the slot
can supply at least X, so advertise X", as opposed to "the slot can
supply X or less, so advertise Y".

Bjorn