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

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Date:   Tue, 24 Aug 2021 10:35:34 -0500
From:   Rob Herring <robh@...nel.org>
To:     Pali Rohár <pali@...nel.org>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Marek Behún <kabel@...nel.org>,
        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, Aug 20, 2021 at 06:00:21PM +0200, Pali Rohár 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(+)

This needs to be in dtschema schemas/pci/pci-bus.yaml instead.

(pci.txt is still here because it needs to be relicensed to move all the 
descriptions to pci-bus.yaml.)

> 
> 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

As mentioned, this should have a unit suffix. I'm not sure it is 
beneficial to share with SFP in this case though.

> +   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.

I no nothing about how this mechanism works, but I think this belongs in 
the next section as for PCIe, a slot is always below a PCI-PCI bridge. 
If we have N slots, then there's N bridges and needs to be N 
slot-power-limit properties, right?

(The same is probably true for all the properties here except 
linux,pci-domain.) There's no distinction between host and PCI bridges  
in pci-bus.yaml though.

Rob