linux-kernel - Re: [PATCH v3 2/6] PCI: brcmstb: Add control of EP voltage regulators

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Date:   Mon, 29 Mar 2021 14:09:58 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Mark Brown <broonie@...nel.org>, Jim Quinlan <jim2101024@...il.com>
Cc:     linux-pci <linux-pci@...r.kernel.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Rob Herring <robh@...nel.org>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        Jim Quinlan <james.quinlan@...adcom.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/6] PCI: brcmstb: Add control of EP voltage regulators

On 3/29/21 1:45 PM, Mark Brown wrote:
> On Mon, Mar 29, 2021 at 03:48:46PM -0400, Jim Quinlan wrote:
> 
>> I'm not concerned about a namespace collision and I don't think you
>> should be concerned either.  First, this driver is for Broadcom STB
>> PCIe chips and boards, and we also deliver the DT to the customers.
>> We typically do not have any other regulators in the DT besides the
>> ones I am proposing.  For example, the 7216 SOC DT has 0 other
> 
> You may not describe these regulators in the DT but you must have other
> regulators in your system, most devices need power to operate.  In any
> case "this works for me with my DT on my system and nobody will ever
> change our reference design" isn't really a great approach, frankly it's
> not a claim I entirely believe and even if it turns out to be true for
> your systems if we establish this as being how regulators work for
> soldered down PCI devices everyone else is going to want to do the same
> thing, most likely making the same claims for how much control they have
> over the systems things will run on.
> 
>> regulators -- no namespace collision possible.  Our DT-generating
>> scripts also flag namespace issues.  I admit that this driver is also
>> used by RPi chips, but I can easily exclude this feature from the RPI
>> if Nicolas has any objection.
> 
> That's certainly an issue, obviously the RPI is the sort of system where
> I'd imagine this would be particularly useful.
> 
>> Further, if you want, I can restrict the search to the two regulators
>> I am proposing to add to pci-bus.yaml:  "vpcie12v-supply" and
>> "vpcie3v3-supply".
> 
> No, that doesn't help - what happens if someone uses separate regulators
> for different PCI devices?  The reason the supplies are device namespaced
> is that each device can look up it's own supplies and label them how it
> wants without reference to anything else on the board.  Alternatively
> what happens if some device has another supply it needs to power on (eg,
> something that wants a clean LDO output for analogue use)?
> 
>> Is the above enough to alleviate your concerns about global namespace collision?
> 
> Not really.  TBH it looks like this driver is keeping the regulators
> enabled all the time except for suspend and resume anyway, if that's all
> that's going on I'd have thought that you wouldn't need any explicit
> management in the driver anyway?  Just mark the regualtors as always on
> and set up an appropriate suspend mode configuration and everything
> should work without the drivers doing anything.  Unless your PMIC isn't
> able to provide separate suspend mode configuration for the regulators?
> 

We have typically GPIO-controlled and PMIC (via SCMI) controlled
regulators. During PCIe enumeration we need these regulators turned on
to be successful in training the PCIe link and discover the end-point
attached, so there an always on regulator would work.

When we enter a system suspend state however there are really two cases:

- the end-point supports Wake-on (typically wake-on-WLAN) and we need
its power supplied kept on to support that

- the end-point does not support or participate in any wake-up, there we
want to turn its supplies off to save power

How would we go about supporting such an use case with an always on
regulator?
-- 
Florian