linux-kernel - Re: [PATCH v2] PCI: Mark LSI FW643 to avoid bus reset

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Message-ID: <ZgZGbMj0I3_6Rt0f@wunner.de>
Date: Fri, 29 Mar 2024 05:41:16 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: Bjorn Helgaas <helgaas@...nel.org>,
	linux1394-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, edmund.raile@...ton.me,
	Takashi Sakamoto <o-takashi@...amocchi.jp>
Subject: Re: [PATCH v2] PCI: Mark LSI FW643 to avoid bus reset

On Thu, Mar 28, 2024 at 02:42:01PM -0600, Alex Williamson wrote:
> On Wed, 27 Mar 2024 10:01:19 -0500 Bjorn Helgaas <helgaas@...nel.org> wrote:
> The original patch proposed for this gave me the impression that this
> was a device used on various old Mac systems, not likely applicable to
> a general purpose plug-in card.

I'm still using one of those "old Mac systems" as my daily driver.

Just checked the ACPI tables and there's an FPEN method below the
FRWR device which toggles GPIO 48 on the PCH.  Checked the schematics
as well and GPIO 48 is marked FW_PWR_EN.  The GPIO controls load
switches which cut power to the FW643 chip when nothing is connected.

Also, FW_PWR_EN feeds into an SLG4AP016V chip where it seems to
internally gate FW_CLKREQ_L.

I'm guessing the driver may need to call the FPEN ACPI method after
issuing a SBR to force the chip on (or perhaps first off, then on)
and thereby re-enable Clock Request.

It's a pity the ohci.c driver doesn't seem to support runtime PM.
That would allow cutting power to the chip when nothing is connected
and thus increase battery life.  The ACPI tables indicate that the
platform sends a notification when something is plugged in, so all
the necessary ingredients are there but we're not taking advantage
of them.

Thanks,

Lukas