linux-kernel - Re: [PATCH v1 1/2] ACPI: EC: Install address space handler at the namespace root

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Date: Fri, 10 May 2024 19:56:45 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Armin Wolf <W_Armin@....de>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, "Rafael J. Wysocki" <rjw@...ysocki.net>, 
	Linux ACPI <linux-acpi@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Hans de Goede <hdegoede@...hat.com>, 
	Mario Limonciello <mario.limonciello@....com>, 
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Subject: Re: [PATCH v1 1/2] ACPI: EC: Install address space handler at the
 namespace root

On Fri, May 10, 2024 at 6:52 PM Armin Wolf <W_Armin@....de> wrote:
>
> Am 10.05.24 um 18:41 schrieb Rafael J. Wysocki:
>
> > On Fri, May 10, 2024 at 6:10 PM Armin Wolf <W_Armin@....de> wrote:
> >> Am 10.05.24 um 16:03 schrieb Rafael J. Wysocki:
> >>
> >>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >>>
> >>> It is reported that _DSM evaluation fails in ucsi_acpi_dsm() on Lenovo
> >>> IdeaPad Pro 5 due to a missing address space handler for the EC address
> >>> space:
> >>>
> >>>    ACPI Error: No handler for Region [ECSI] (000000007b8176ee) [EmbeddedControl] (20230628/evregion-130)
> >>>
> >>> This happens because the EC driver only registers the EC address space
> >>> handler for operation regions defined in the EC device scope of the
> >>> ACPI namespace while the operation region being accessed by the _DSM
> >>> in question is located beyond that scope.
> >>>
> >>> To address this, modify the ACPI EC driver to install the EC address
> >>> space handler at the root of the ACPI namespace.
> >>>
> >>> Note that this change is consistent with some examples in the ACPI
> >>> specification in which EC operation regions located outside the EC
> >>> device scope are used (for example, see Section 9.17.15 in ACPI 6.5),
> >>> so the current behavior of the EC driver is arguably questionable.
> >> Hi,
> >>
> >> the patch itself looks good to me, but i wonder what happens if multiple
> >> ACPI EC devices are present. How would we handle such a situation?
> > I'm wondering if this is a theoretical question or do you have any
> > existing or planned systems in mind?
> >
> > ec_read(), ec_write() and ec_transaction() use only the first EC that
> > has been found anyway.
>
> Its a theoretical question, i do not know of any systems which have more than
> one ACPI EC device.
>
> This patch would prevent any ACPI ECs other than the first one from probing,
> since they would fail to register their address space handler.
> I am just curious if/how we want to handle such situations.

I'm not worried until I see a system where that is a problem.

That said, it can be addressed by adding a first_ec check around the
address space handler registration/unregistration.