linux-kernel - Re: [PATCH] of: property: Create devlink between PCI Host bridge and Root Port suppliers

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Message-ID: <lnzmgsdjckbf6u5hxfqm4kzlmcg2cqvszcg7otmkqxaj3yzdfq@vu232xx5kts3>
Date: Thu, 5 Feb 2026 14:31:31 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: robh@...nel.org, saravanak@...nel.org, andersson@...nel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, qiang.yu@....qualcomm.com
Subject: Re: [PATCH] of: property: Create devlink between PCI Host bridge and
 Root Port suppliers

On Thu, Feb 05, 2026 at 09:50:20AM +0100, Konrad Dybcio wrote:
> On 2/5/26 8:06 AM, Manivannan Sadhasivam wrote:
> > In the recent times, devicetree started to represent the PCI Host bridge
> > supplies like PHY in the Root Port nodes as seen in commit 38fcbfbd4207
> > ("dt-bindings: PCI: qcom: Move PHY & reset GPIO to Root Port node"). But
> > the Host bridge drivers still need to control these supplies as a part of
> > their controller initialization/deinitialization sequence.
> > 
> > So the Host bridge drivers end up parsing the Root Port supplies in their
> > probe() and controlled them. A downside to this approach is that the
> > devlink dependency between the suppliers and Host bridge is completely
> > broken. Due to this, the driver core probes the Host bridge drivers even if
> > the suppliers are not ready, causing probe deferrals and setup teardowns in
> > probe().
> > 
> > These probe deferrals sometime happen over 1000 times (as reported in Qcom
> > Glymur platform) leading to a waste of CPU resources and increase in boot
> > time. So to fix these unnecessary deferrals, create devlink between the
> > Host bridge and Root Port suppliers in of_fwnode_add_links(). This will
> > allow the driver core to probe the Host bridge drivers only when all Root
> > Port suppliers are available.
> > 
> > Reported-by: Bjorn Andersson <andersson@...nel.org>
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>
> > ---
> 
> [...]
> 
> This is not 'required' in bindings and device_type="pci" doesn't uniquely
> identify root complexes (as can be seen below).. but I suppose this is the
> best delimiter we've got
> 

Yeah. There is no way to uniquely identify the Host bridges in DT. So I had to
settle for this.

Maybe I can check for 'device_type', but that will create devlink between switch
port supplies and Root Ports.

> Perhaps it could be made 'required'?
> 

Nah. Linux will generate domain numbers on its own. Also, this is a Linux
specific property, so we cannot make it mandatory in dtschema.

- Mani

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