linux-kernel - Re: [PATCH v2 2/6] PCI: brcmstb: Split brcm_pcie

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Message-ID: <CA+-6iNwjPr2gu_oyn4NheLPJZHh-3eib-3onz63sfNOJpdJ6Tw@mail.gmail.com>
Date:   Mon, 18 Jul 2022 14:56:03 -0400
From:   Jim Quinlan <james.quinlan@...adcom.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Jim Quinlan <jim2101024@...il.com>,
        "open list:PCI NATIVE HOST BRIDGE AND ENDPOINT DRIVERS" 
        <linux-pci@...r.kernel.org>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Cyril Brulebois <kibi@...ian.org>,
        "maintainer:BROADCOM BCM7XXX ARM ARCHITECTURE" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.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 v2 2/6] PCI: brcmstb: Split brcm_pcie_setup() into two funcs

On Mon, Jul 18, 2022 at 2:14 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> On Sat, Jul 16, 2022 at 06:24:49PM -0400, Jim Quinlan wrote:
> > Currently, the function does the setup for establishing PCIe link-up
> > with the downstream device, and it does the actual link-up as well.
> > The calling sequence is (roughly) the following in the probe:
> >
> > -> brcm_pcie_probe()
> >     -> brcm_pcie_setup();                       /* Set-up and link-up */
> >     -> pci_host_probe(bridge);
> >
> > This commit splits the setup function in two: brcm_pcie_setup(), which only
> > does the set-up, and brcm_pcie_start_link(), which only does the link-up.
> > The reason why we are doing this is to lay a foundation for subsequent
> > commits so that we can turn on any power regulators, as described in the
> > root port's DT node, prior to doing link-up.
>
> All drivers that care about power regulators turn them on before
> link-up, but typically those regulators are described directly under
> the host bridge itself.
Hi Bjorn,

Actually, what you describe is what I proposed with my v1 back in Nov 2020.
The binding commit message said,

    "Quite similar to the regulator bindings found in "rockchip-pcie-host.txt",
     this allows optional regulators to be attached and controlled by the
     PCIe RC driver."

>
> IIUC the difference here is that you have regulators described under
> Root Ports (not the host bridge/Root Complex itself), so you don't
> know about them until you've enumerated the Root Ports.
> brcm_pcie_probe() can't turn them on directly because it doesn't know
> what Root Ports are present and doesn't know about regulators below
> them.

The reviewer's requested me to move the regulator node(s)  elsewhere,
and at some point later it was requested to be placed under the Root Port
driver.  I would love to return them under the host bridge, just say the word!

>
> So I think brcm_pcie_setup() does initialization that doesn't depend
> on the link or any downstream devices, and brcm_pcie_start_link() does
> things that depend on the link being up.  Right?
Yes.

>
> If so, "start_link" might be a slight misnomer since AFAICT
> brcm_pcie_start_link() doesn't do anything to initiate link-up except
> maybe deasserting fundamental reset.  Some drivers start the LTSSM or
> explicitly enable link training, but brcm_pcie_start_link() doesn't
> seem to do anything like that.
>
> brcm_pcie_start_link() still does brcm_pcie_set_outbound_win().  Does
> that really depend on the link being up?  If that only affects the
> Root Port, maybe it could be done before link-up?
Some of the registers cannot be accessed until after linkup but these do
not have that issue.  I will change this.

Jim Quinlan
Broadcom STB

>
> > We do this by defining an
> > add_bus() callback which is invoked during enumeraion.  At the end of this
> > patchset the probe function trace will look something like this:
> >
> > -> brcm_pcie_probe()
> >     -> brcm_pcie_setup();                       /* Set-up only */
> >     -> pci_host_probe(bridge);
> >         -> [enumeration]
> >             -> pci_alloc_child_bus()
> >                 -> bus->ops->add_bus(bus);      /* We've set this op */
> >                 -> brcm_pcie_add_bus()          /* Our callback      */
> >                      -> [turn on regulators]    /* Main objective!   */
> >                      -> brcm_pcie_start_link()  /* Link-up           */

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