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

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Message-ID: <20210824161409.2mxzpy5r32tm3kgu@pali>
Date:   Tue, 24 Aug 2021 18:14:09 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Rob Herring <robh@...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 Tuesday 24 August 2021 10:35:34 Rob Herring wrote:
> 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.)

Ok, this is just a proposal for a new DTS property. So documentation
issues will be fixed in real patch.

> > 
> > 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

This slot-power-limit property belongs to same place where are also
other slot properties (link speed, reset gpios, clkreq). So I put it in
place where others are.

But I'm not sure where it should be as it affects link/slot. Because
link has two sides. I guess that link speed and slot power limit could
belong to the root/downstream port and reset gpio to the endpoint card
or upstream port...