linux-kernel - Re: [PATCH v3 2/7] regulator: qca6390: add support for QCA639x powerup sequence

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Date:   Wed, 14 Jul 2021 12:10:10 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Peter Chen <peter.chen@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-bluetooth@...r.kernel.org
Subject: Re: [PATCH v3 2/7] regulator: qca6390: add support for QCA639x
 powerup sequence

On Wed 14 Jul 11:47 CDT 2021, Rob Herring wrote:

> On Thu, Jul 08, 2021 at 02:37:44PM +0300, Dmitry Baryshkov wrote:
> > Hi,
> > 
> > On Thu, 8 Jul 2021 at 13:10, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > >
> > > - Peter (the email was bouncing)
> > 
> > + Peter's kernel.org address
> > 
> > >
> > > On Tue, 6 Jul 2021 at 13:55, Mark Brown <broonie@...nel.org> wrote:
> > > >
> > > > On Tue, Jul 06, 2021 at 09:54:03AM +0200, Ulf Hansson wrote:
> > > > > On Tue, 22 Jun 2021 at 00:32, Dmitry Baryshkov
> > > >
> > > > > > Qualcomm QCA6390/1 is a family of WiFi + Bluetooth SoCs, with BT part
> > > > > > being controlled through the UART and WiFi being present on PCIe
> > > > > > bus. Both blocks share common power sources. Add device driver handling
> > > > > > power sequencing of QCA6390/1.
> > > >
> > > > > Power sequencing of discoverable buses have been discussed several
> > > > > times before at LKML. The last attempt [1] I am aware of, was in 2017
> > > > > from Peter Chen. I don't think there is a common solution, yet.
> > > >
> > > > This feels a bit different to the power sequencing problem - it's not
> > > > exposing the individual inputs to the device but rather is a block that
> > > > manages everything but needs a bit of a kick to get things going (I'd
> > > > guess that with ACPI it'd be triggered via AML).  It's in the same space
> > > > but it's not quite the same issue I think, something that can handle
> > > > control of the individual resources might still struggle with this.
> > >
> > > Well, to me it looks very similar to those resouses we could manage
> > > with the mmc pwrseq, for SDIO. It's also typically the same kind of
> > > combo-chips that moved from supporting SDIO to PCIe, for improved
> > > performance I guess. More importantly, the same constraint to
> > > pre-power on the device is needed to allow it to be discovered/probed.
> > 
> > In our case we'd definitely use pwrseq for PCIe bus and we can also
> > benefit from using pwrseq for serdev and for platform busses also (for
> > the same story of WiFi+BT chips).
> > 
> > I can take a look at rewriting pwrseq code to also handle the PCIe
> > bus. Rewriting it to be a generic lib seems like an easy task,
> > plugging it into PCIe code would be more fun.
> > 
> > Platform and serdev... Definitely even more fun.
> 
> I don't want to see pwrseq (the binding) expanded to other buses. If 
> that was the answer, we wouldn't be having this discussion. It was a 
> mistake for MMC IMO. 
> 

But what do you want to see?

We have a single piece of hardware that needs a specific power sequence,
which is interacted with using both UART and PCIe.

> If pwrseq works as a kernel library/api, then I have no issue with that.
> 
> > 
> > > Therefore, I think it would be worth having a common solution for
> > > this, rather than a solution per subsystem or even worse, per device.
> 
> Power sequencing requirements are inheritently per device unless we're 
> talking about standard connectors. 
> 

Do you mean "device" as in the IC or device as in struct device? Because
we do have one physical IC, that has a need for a specific power on
sequence, but we have two struct device, on two different busses in
Linux interacting with this thing.

> This is a solved problem on MDIO. It's quite simple. If there's a DT 
> node for a device you haven't discovered, then probe it anyways.
> 

Okay, so DT tells us that there's actually a WiFi thing on the PCIe bus,
even though we can't find it, so we probe something...then what?

Regards,
Bjorn