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SubjectRe: [PATCH 5/5] ACPI: Tie ACPI backlight devices to PCI devices if possible
Date
On Friday, January 14, 2011, Matthew Garrett wrote:
> Dual-GPU machines may provide more than one ACPI backlight interface. Tie
> the backlight device to the GPU in order to allow userspace to identify
> the correct interface.
>
> Signed-off-by: Matthew Garrett <mjg@redhat.com>

Sorry for the late response, but I've just realized there's a problem with this
patch.

> ---
> drivers/acpi/video.c | 15 ++++++++++++++-
> 1 files changed, 14 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
> index a9eec8c..a18e497 100644
> --- a/drivers/acpi/video.c
> +++ b/drivers/acpi/video.c
> @@ -782,6 +782,9 @@ static void acpi_video_device_find_cap(struct acpi_video_device *device)
>
> if (acpi_video_backlight_support()) {
> struct backlight_properties props;
> + struct pci_dev *pdev;
> + acpi_handle acpi_parent;
> + struct device *parent = NULL;
> int result;
> static int count = 0;
> char *name;
> @@ -794,10 +797,20 @@ static void acpi_video_device_find_cap(struct acpi_video_device *device)
> return;
> count++;
>
> + acpi_get_parent(device->dev->handle, &acpi_parent);
> +
> + pdev = acpi_get_pci_dev(acpi_parent);
> + if (pdev) {
> + parent = &pdev->dev;
> + pci_dev_put(pdev);
> + }

I'm afraid you can't do that or suspend problems will happen.

Namely, the PM core assumes that parents will be registered before their
children and it is a very strong assumption. If violated, it may lead to
a suspend sequence deadlock.

Unfortunately, it looks like the ACPI video device is registered before the
PCI device that's becoming it's parent, so the above assumption will be
violated.

Thanks,
Rafael


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