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 20:50:28 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Mario Limonciello <mario.limonciello@....com>
Cc: 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 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.

> > > 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.

-- 
With Best Regards,
Andy Shevchenko