Re: [PATCH] ACPI / PM: Traverse video_device_list for backlightrestoration

From: Zhang Rui
Date: Tue Aug 03 2010 - 21:24:09 EST

On Tue, 2010-08-03 at 18:11 +0800, Matthias Hopf wrote:
> A bit of background about this patch:
> acpi_video_bus_get_devices() explicitly calls acpi_video_device_enumerate()
> before acpi_video_bus_get_one_device(), which does acpi_video_device_bind().
> In _enumerate(), active_list[] is created, with .bind_info explicitly set to
> NULL, while the related .bind_info entry in video_device_list is set to the
> acpi_video_device ptr in _bind().
> The .bind_info is later used in acpi_video_resume() to re-set the backlight
> - but it's only evaluated on the active_list[], on which all .bind_info are
> NULL by construction.

why? .bind_info in active_list is initialized in acpi_video_device_bind.

> This results in backlight not being restored, if the
> BIOS doesn't do this by itself already.
> The patch resolves this by replacing the active_list[] traversal by a
> video_device_list traversal,

> but due to the nature of the issue I'm unsure
> whether the original issue isn't of more principal quality.

> I don't understand
> why the list is (partially) transformed into an array in the first place,
> especially as both the array *and* the list are used in the code...
no, the active_list is an array in fact, which equals
acpi_video_bus->attached_array. Maybe we need to change it to a more
proper name.

I'm still wondering why the patch works for you.
could you please attach the acpidump output of this laptop?

please rebuild your kernel with CONFIG_ACPI_DEBUG=y, and reboot with
kernel parameter "acpi.debug_layer=0x10 acpi.debug_level=0x400".
and attach the dmesg output after resume, both w/ and w/o the patch you


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