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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Sun, 22 Apr 2018 11:34:06 +0200
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Subject: Re: [PATCH] ACPI / scan: Fix regression related to X-Gene UARTs
To:     Mark Salter <msalter@redhat.com>
Cc:     =?UTF-8?B?RnLDqWTDqXJpYyBEYW5pcw==?= <frederic.danis.oss@gmail.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Apr 20, 2018 at 5:29 AM, Mark Salter <msalter@redhat.com> wrote:
> Commit e361d1f85855 ("ACPI / scan: Fix enumeration for special UART
> devices") caused a regression with some X-Gene based platforms (Mustang
> and M400) with invalid DSDT.

I'm not convinced that making changes to the core ACPI device
enumeration code in order to cover up for firmware bugs is the right
approach.

> The DSDT makes it appear that the UART
> device is also a slave device attached to itself. With the above commit
> the UART won't be enumerated by ACPI scan (slave serial devices shouldn't
> be). So check for X-Gene UART device and skip slace device check on it.
>
> Signed-off-by: Mark Salter <msalter@redhat.com>
> ---
>  drivers/acpi/scan.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
> index cc234e6a6297..1dcdd0122862 100644
> --- a/drivers/acpi/scan.c
> +++ b/drivers/acpi/scan.c
> @@ -1551,6 +1551,14 @@ static bool acpi_device_enumeration_by_parent(struct acpi_device *device)
>              fwnode_property_present(&device->fwnode, "baud")))
>                 return true;
>
> +       /*
> +        * Firmware on some arm64 X-Gene platforms will make the UART
> +        * device appear as both a UART and a slave of that UART. Just
> +        * bail out here for X-Gene UARTs.
> +        */
> +       if (!strcmp(acpi_device_hid(device), "APMC0D08"))
> +               return false;

Is the device ID never to be used outside of the broken configurations?

Even if that's the plan, how are you going to guarantee that anyway?

> +
>         INIT_LIST_HEAD(&resource_list);
>         acpi_dev_get_resources(device, &resource_list,
>                                acpi_check_serial_bus_slave,
> --