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Date:   Tue, 2 Feb 2021 15:50:53 -0600
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Rob Herring <robh+dt@kernel.org>
Cc:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Andy Gross <agross@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Stanimir Varbanov <svarbanov@mm-sol.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        PCI <linux-pci@vger.kernel.org>
Subject: Re: [PATCH v2 3/5] pcie-qcom: provide a way to power up qca6390 chip
 on RB5 platform
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On Tue 02 Feb 15:37 CST 2021, Rob Herring wrote:

> On Tue, Feb 2, 2021 at 1:48 PM Bjorn Andersson
> <bjorn.andersson@linaro.org> wrote:
> >
> > On Sat 30 Jan 10:14 CST 2021, Dmitry Baryshkov wrote:
> >
> > > On Sat, 30 Jan 2021 at 06:53, Bjorn Andersson
> > > <bjorn.andersson@linaro.org> wrote:
> > > >
> > > > On Fri 29 Jan 16:19 CST 2021, Dmitry Baryshkov wrote:
> > > >
> > > > > On Sat, 30 Jan 2021 at 00:50, Bjorn Helgaas <helgaas@kernel.org> wrote:
> > > > > >
> > > > > > On Fri, Jan 29, 2021 at 06:45:21AM +0300, Dmitry Baryshkov wrote:
> > > > > > > On 28/01/2021 22:26, Rob Herring wrote:
> > > > > > > > On Thu, Jan 28, 2021 at 11:52 AM Dmitry Baryshkov
> > > > > > > > <dmitry.baryshkov@linaro.org> wrote:
> > > > > > > > >
> > > > > > > > > Some Qualcomm platforms require to power up an external device before
> > > > > > > > > probing the PCI bus. E.g. on RB5 platform the QCA6390 WiFi/BT chip needs
> > > > > > > > > to be powered up before PCIe0 bus is probed. Add a quirk to the
> > > > > > > > > respective PCIe root bridge to attach to the power domain if one is
> > > > > > > > > required, so that the QCA chip is started before scanning the PCIe bus.
> > > > > > > >
> > > > > > > > This is solving a generic problem in a specific driver. It needs to be
> > > > > > > > solved for any PCI host and any device.
> > > > > > >
> > > > > > > Ack. I see your point here.
> > > > > > >
> > > > > > > As this would require porting code from powerpc/spark of-pci code and
> > > > > > > changing pcie port driver to apply power supply before bus probing happens,
> > > > > > > I'd also ask for the comments from PCI maintainers. Will that solution be
> > > > > > > acceptable to you?
> > > > > >
> > > > > > I can't say without seeing the code.  I don't know enough about this
> > > > > > scenario to envision how it might look.
> > > > > >
> > > > > > I guess the QCA6390 is a PCIe device?  Why does it need to be powered
> > > > > > up before probing?  Shouldn't we get a link-up interrupt when it is
> > > > > > powered up so we could probe it then?
> > > > >
> > > > > Not quite. QCA6390 is a multifunction device, with PCIe and serial
> > > > > parts. It has internal power regulators which once enabled will
> > > > > powerup the PCIe, serial and radio parts. There is no need to manage
> > > > > regulators. Once enabled they will automatically handle device
> > > > > suspend/resume, etc.
> > > > >
> > > >
> > > > So what you're saying is that if either the PCI controller or bluetooth
> > > > driver probes these regulators will be turned on, indefinitely?
> > > >
> > > > If so, why do we need a driver to turn them on, rather than just mark
> > > > them as always-on?
> > > >
> > > > What's the timing requirement wrt regulators vs WL_EN/BT_EN?
> > >
> > > According to the documentation I have, they must be enabled right
> > > after enabling powering the chip and they must stay enabled all the
> > > time.
> > >
> >
> > So presumably just marking these things always-on and flipping the GPIO
> > statically won't be good enough due to the lack of control over the
> > timing.
> >
> > This really do look like a simplified case of what we see with the
> > PCIe attached modems, where similar requirements are provided, but also
> > the ability to perform a device specific reset sequence in case the
> > hardware has locked up. I'm slightly worried about the ability of
> > extending your power-domain model to handle the restart operation
> > though.
> 
> I think this is an abuse of 'power-domains'. Just define the
> regulators in both WiFi and BT nodes and have each driver enable them.
> They're refcounted. If that's still not enough control over the power
> sequencing, then create a 3rd entity to do it, but that doesn't need
> to leak into DT. You already have all the information you need.
> 

As Dmitry explained he still need to pull the two GPIOs high after
enabling the regulators, but before scanning the PCI or serdev buses.

I was thinking something along the lines you suggest, but I've not been
able to come up with something that will guarantee the ordering of the
events.

Regards,
Bjorn