linux-kernel - Re: [RFC PATCH 1/3] dt-bindings: Add 'slot-power-limit' PCIe port property

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Date:   Sun, 22 Aug 2021 14:38:43 +0200
From:   Marek Behún <kabel@...nel.org>
To:     Pali Rohár <pali@...nel.org>
Cc:     Rob Herring <robh@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] dt-bindings: Add 'slot-power-limit' PCIe port
 property

On Fri, 20 Aug 2021 18:00:21 +0200
Pali Rohár <pali@...nel.org> wrote:

> This property specifies slot power limit in mW unit. It is form-factor and
> board specific value and must be initialized by hardware.
> 
> Some PCIe controllers delegates this work to software to allow hardware
> flexibility and therefore this property basically specifies what should
> host bridge programs into PCIe Slot Capabilities registers.
> 
> Property needs to be specified in mW unit, and not in special format
> defined by Slot Capabilities (which encodes scaling factor or different
> unit). Host drivers should convert value from mW unit to their format.
> 
> Signed-off-by: Pali Rohár <pali@...nel.org>
> ---
>  Documentation/devicetree/bindings/pci/pci.txt | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/pci/pci.txt b/Documentation/devicetree/bindings/pci/pci.txt
> index 6a8f2874a24d..e67d5db21514 100644
> --- a/Documentation/devicetree/bindings/pci/pci.txt
> +++ b/Documentation/devicetree/bindings/pci/pci.txt
> @@ -32,6 +32,12 @@ driver implementation may support the following properties:
>     root port to downstream device and host bridge drivers can do programming
>     which depends on CLKREQ signal existence. For example, programming root port
>     not to advertise ASPM L1 Sub-States support if there is no CLKREQ signal.
> +- slot-power-limit:
> +   If present this property specifies slot power limit in mW unit. Host drivers
> +   can parse this slot power limit and use it for programming Root Port or host
> +   bridge, or for composing and sending PCIe Set_Slot_Power_Limit message
> +   through the Root Port or host bridge when transitioning PCIe link from a
> +   non-DL_Up Status to a DL_Up Status.
>  
>  PCI-PCI Bridge properties
>  -------------------------

SFP uses something similar for maximum power, but since the units are
in milliwatt, the name of the property is maximum-power-milliwatt, to
avoid problems. I think it should be done here as well, i.e.
  slot-maximum-power-milliwatt
or maybe even
  maximum-power-milliwatt.

Marek