linux-kernel - Re: [PATCH v2] schemas: pci: Document PCIe T_POWER

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Message-ID: <enri4affdgq4q5mibnmhldhqqoybqbdcswohoj5mst2i77ckmu@dwlaqfxyjy3w>
Date: Thu, 13 Nov 2025 10:01:32 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Lukas Wunner <lukas@...ner.de>
Cc: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>, 
	andersson@...nel.org, robh@...nel.org, manivannan.sadhasivam@...aro.org, 
	krzk@...nel.org, helgaas@...nel.org, linux-arm-msm@...r.kernel.org, 
	devicetree@...r.kernel.org, lpieralisi@...nel.org, kw@...ux.com, conor+dt@...nel.org, 
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org, devicetree-spec@...r.kernel.org, 
	quic_vbadigan@...cinc.com
Subject: Re: [PATCH v2] schemas: pci: Document PCIe T_POWER_ON

On Mon, Nov 10, 2025 at 01:41:45PM +0100, Lukas Wunner wrote:
> On Mon, Nov 10, 2025 at 04:59:47PM +0530, Krishna Chaitanya Chundru wrote:
> > From PCIe r6, sec 5.5.4 & Table 5-11 in sec 5.5.5 T_POWER_ON is the
> 
> Please use the latest spec version as reference, i.e. PCIe r7.0.
> 
> > minimum amount of time(in us) that each component must wait in L1.2.Exit
> > after sampling CLKREQ# asserted before actively driving the interface to
> > ensure no device is ever actively driving into an unpowered component and
> > these values are based on the components and AC coupling capacitors used
> > in the connection linking the two components.
> > 
> > This property should be used to indicate the T_POWER_ON for each Root Port.
> 
> What's the difference between this property and the Port T_POWER_ON_Scale
> and T_POWER_ON_Value in the L1 PM Substates Capabilities Register?
> 
> Why do you need this in the device tree even though it's available
> in the register?
> 

Someone needs to program these registers. In the x86 world, BIOS will do it
happily, but in devicetree world, OS has to do it. And since this is a platform
specific value, this is getting passed from devicetree.

- Mani

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