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

Bjorn Helgaas
2022-02-24T20:48:35Z

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).

> + 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?

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.

> +}
> +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