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

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Date:   Wed, 14 Jul 2021 10:47:10 -0600
From:   Rob Herring <robh@...nel.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Peter Chen <peter.chen@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.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 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. 

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. 

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.

Rob