linux-kernel - Re: [PATCH v2 2/2] PCI: hotplug: Add extension driver for Ampere Altra hotplug LED control

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Message-ID: <20231025172237.GA1754650@bhelgaas>
Date:   Wed, 25 Oct 2023 12:22:37 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     D Scott Phillips <scott@...amperecomputing.com>
Cc:     linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
        Darren Hart <darren@...amperecomputing.com>,
        patches@...erecomputing.com
Subject: Re: [PATCH v2 2/2] PCI: hotplug: Add extension driver for Ampere
 Altra hotplug LED control

On Fri, Sep 29, 2023 at 05:20:36PM -0700, D Scott Phillips wrote:
> On Ampere Altra, PCIe hotplug is handled through ACPI. A side interface is
> also present to request system firmware control of attention LEDs. Add an
> ACPI PCI Hotplug companion driver to support attention LED control.
> ...

> +	arm_smccc_smc(REQUEST, LED_CMD, led_status(status), LED_ATTENTION,
> +		      pci_domain_nr(bus) | (PCI_SLOT(root_port->devfn) << 4), 0, 0,

pci_domain_nr() returns "int" (normally 32 bits), but since this is an
ACPI system, the domain comes from _SEG, which is defined to be 16
bits (ACPI r6.5, sec 6.5.6).

So it looks like ORing in the "slot << 4" clobbers the upper 12 bits
of _SEG.

Is this code doing the right thing?