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Date: Mon, 11 Apr 2022 13:14:07 +0200
From: Pali Rohár <pali@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
Bjorn Helgaas <bhelgaas@...gle.com>,
Rob Herring <robh+dt@...nel.org>, Andrew Lunn <andrew@...n.ch>,
Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
Krzysztof Wilczyński <kw@...ux.com>,
Marek Behún <kabel@...nel.org>,
Russell King <rmk+kernel@...linux.org.uk>,
linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/4] PCI: Add function for parsing
'slot-power-limit-milliwatt' DT property
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@...nel.org>
> > Signed-off-by: Marek Behún <kabel@...nel.org>
> > Reviewed-by: Rob Herring <robh@...nel.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 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
> >
> >
> > _______________________________________________
> > linux-arm-kernel mailing list
> > linux-arm-kernel@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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