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 20:06:11 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Mario Limonciello <mario.limonciello@....com>, Armin Wolf <W_Armin@....de>, 
	"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>, 
	Hans de Goede <hdegoede@...hat.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 7:50 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Fri, May 10, 2024 at 12:40:05PM -0500, Mario Limonciello wrote:
> > On 5/10/2024 12:29, Andy Shevchenko wrote:
> > > On Fri, May 10, 2024 at 06:52:41PM +0200, Armin Wolf 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.
> > >
> > > The specification is clear about this case in the "ACPI Embedded Controller
> > > Interface Specification":
> > >
> > >   "The ACPI standard supports multiple embedded controllers in a system,
> > >    each with its own resources. Each embedded controller has a flat
> > >    byte-addressable I/O space, currently defined as 256 bytes."
> > >
> > > However, I haven't checked deeper, so it might be a leftover in the documentation.
> > >
> > > The OperationRegion() has no reference to the EC (or in general, device) which
> > > we need to speak to. The only possibility to declare OpRegion() for the second+
> > > EC is to use vendor specific RegionSpace, AFAIU. So, even if ACPI specification
> > > supports 2+ ECs, it doesn't support OpRegion():s for them under the same
> > > RegionSpace.
> > >
> > > That said, the commit message might be extended to summarize this, but at
> > > the same time I see no way how this series can break anything even in 2+ ECs
> > > environments.
> >
> > It's deviating from the patch, but in practice /why/ would you even want to
> > have a design with two ECs?  In general that is going to mean a much more
> > complex state machine with synchronizing the interaction between both of
> > them and the host.
> >
> > Understanding the benefit of such a design might make it easier to
> > hypothesize impacts.
>
> First that comes to my mind (but hypothetical), is the separate CPU/EC add-on
> cards. If the main firmware somehow supports all of these add-on platforms,
> it might need to handle 2+ ECs.
>
> Again, it might be ACPI specification issue. For instance, the cited piece
> doesn't tell about 16-bit EC accesses.

IMV this is a matter of what is testable.

We can only seriously say that we support 1 EC in the system, because
that's what we can test.

Now, the specification allows (theoretically) multiple ECs to be
supported which does not mean that it will ever be done in practice
and it also does not mean that this is a good idea.