linux-kernel - Re: [PATCH v3 4/9] PCI/PM: Rework changing power states of PCI devices

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Date:   Wed, 4 May 2022 19:15:24 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Anders Roxell <anders.roxell@...aro.org>
Subject: Re: [PATCH v3 4/9] PCI/PM: Rework changing power states of PCI devices

On Wed, May 4, 2022 at 7:03 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> [+cc Anders]
>
> On Tue, May 03, 2022 at 10:59:43AM -0700, Nathan Chancellor wrote:
> > On Thu, Apr 14, 2022 at 03:11:21PM +0200, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > >
> > > There are some issues related to changing power states of PCI
> > > devices, mostly related to carrying out unnecessary actions in some
> > > places, and the code is generally hard to follow.
> > >
> > >  1. pci_power_up() has two callers, pci_set_power_state() and
> > >     pci_pm_default_resume_early().  The latter updates the current
> > >     power state of the device right after calling pci_power_up()
> > >     and it restores the entire config space of the device right
> > >     after that, so pci_power_up() itself need not read the
> > >     PCI_PM_CTRL register or restore the BARs after programming the
> > >     device into D0 in that case.
> > >
> > >  2. It is generally hard to get a clear view of the pci_power_up()
> > >     code flow, especially in some corner cases, due to all of the
> > >     involved PCI_PM_CTRL register reads and writes occurring in
> > >     pci_platform_power_transition() and in pci_raw_set_power_state(),
> > >     some of which are redundant.
> > >
> > >  3. The transitions from low-power states to D0 and the other way
> > >     around are unnecessarily tangled in pci_raw_set_power_state()
> > >     which causes it to use a redundant local variable and makes it
> > >     rather hard to follow.
> > >
> > > To address the above shortcomings, make the following changes:
> > >
> > >  a. Remove the code handling transitions into D0
> > >     from pci_raw_set_power_state() and rename it as
> > >     pci_set_low_power_state().
> > >
> > >  b. Add the code handling transitions into D0 directly
> > >     to pci_power_up() and to a new wrapper function
> > >     pci_set_full_power_state() calling it internally that is
> > >     only used in pci_set_power_state().
> > >
> > >  c. Make pci_power_up() avoid redundant PCI_PM_CTRL register reads
> > >     and make it work in the same way for transitions from any
> > >     low-power states (transitions from D1 and D2 are handled
> > >     slightly differently before the change).
> > >
> > >  d. Put the restoration of the BARs and the PCI_PM_CTRL
> > >     register read confirming the power state change into
> > >     pci_set_full_power_state() to avoid doing that in
> > >     pci_pm_default_resume_early() unnecessarily.
> > >
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> >
> > This change as commit 5bffe4c611f5 ("PCI/PM: Rework changing power
> > states of PCI devices") causes my AMD-based system to fail to fully
> > boot.
>
> I dropped 5bffe4c611f5 and subsequent pci/pm patches temporarily while
> this gets worked out.

OK

It looks like I missed something subtle that triggers on a subset of
systems only.