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 11:54:09 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     "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

[+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.

Bjorn