linux-kernel - Re: [PATCH v7 2/3] PCI: brcmstb: Configure HW CLKREQ# mode appropriate for downstream device

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Date:   Thu, 9 Nov 2023 16:31:40 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jim Quinlan <james.quinlan@...adcom.com>
Cc:     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>,
        Phil Elwell <phil@...pberrypi.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Jim Quinlan <jim2101024@...il.com>,
        Florian Fainelli <florian.fainelli@...adcom.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        "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 v7 2/3] PCI: brcmstb: Configure HW CLKREQ# mode
 appropriate for downstream device

On Thu, Nov 09, 2023 at 05:06:15PM -0500, Jim Quinlan wrote:
> ...

> BTW, besides the RPi4, I haven't been able to find a Linux platform
> where I can do
> 
>         echo $POLICY > /sys/module/pcie_aspm/parameters/policy

This sounds like something we should fix.  What exactly happens?  I
think this should be handled at pcie_aspm_set_policy(), so:

  /sys/module/pcie_aspm/parameters/policy doesn't exist (seems
  unlikely)?

  Returns -EPERM (would indicate aspm_disabled)?

  Returns -EINVAL (would indicate $POLICY doesn't match anything in
  policy_str[])?

  Returns 0 with no action (would indicate $POLICY is the same as the
  current aspm_policy)?

> It seems that the FW/ACPI typically locks this down.  I did see a
> comment somewhere that
> said that the reason it was locked down is because too many devices
> cannot handle it.

Do you have any details about FW/ACPI locking this down?
aspm_disabled is set by the kernel "pcie_aspm=off" parameter (I assume
you're not referring to this), if the FADT has ACPI_FADT_NO_ASPM set,
or if a host bridge's _OSC is missing or failed (maybe [1] is the
comment you saw?)

These all *should* be unusual cases, so I'd be surprised if you're
tripping over one of these.  I would NOT be surprised if we had some
issue in pcie_config_aspm_link() or pcie_set_clkpm() that meant the
policy change didn't work as intended, though.

Bjorn

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/acpi/pci_root.c?id=v6.6#n617