(Resending in text-only sorry for the noise to the individual recipients)
Comments below.
-------- Original Message --------
Subject: Re: [PATCH][ACPI] AC/DC notifier
From: Pavel Machek <pavel@xxxxxx>
To: Tippett, Matthew <matthew.tippett@xxxxxxx>
Cc: "Willy Tarreau" <w@xxxxxx>, "Matthew Garrett" <mjg59@xxxxxxxxxxxxx>,
"Langsdorf, Mark" <mark.langsdorf@xxxxxxx>, lenb@xxxxxxxxxx,
linux-acpi@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, "Li, Samuel"
<Samuel.Li@xxxxxxx>
Date: 10/07/2009 03:31 AM
On Tue 2009-10-06 10:53:22, Tippett, Matthew wrote:
Please do. So far you did not show valid use for such notifier.
(Ok, I know of one. Old amd64 notebooks had cpufreq scaling enabled,
with battery unable to supply enough current to feed the CPU at
highest cpufreq setting. At that point, scaling cpufreq down at unplug
is correctness issue, and AC/DC notifier in kernel makes
sense.)
So... what do you want to use it for?
We have a general requirement from OEMs and consequently our shared
Windows/Linux components that the AC/DC state is accurately known.
The concrete examples of use include at least the following.
1) Automatic frequency scaling has an AC-mode and a DC-mode in
Powerplay tables in the GPU BIOS ensures that the highest permitted
clocks fit the system design. This allows at least
i) system level thermals and power consumption to be managed
(eg: you shouldn't have the clocks up high if the system fan has been
asked to slow down).
ii) protection of hardware high clocks with a low-current
battery is a bad idea.
2) The pixel clock can only drive certain modes with certain engine
and memory clocks, in DC-mode you will have lower clocks and
consequently you will need to change the pixel clock and hence the mode
to something that fits within the budget, otherwise the 3D or display
engine may not be able to get the bandwidth required to operate effectively.
3) OEM OS equivalency. Some OEMs care that Linux has the same
thermal, power and performance characteristics as other operating
systems. This requires that the software act in a similar way and
reduces OEM/ODM/IHV/OSV validation and deployment costs.
In particular 1 and 2 are very relevant for KMS based drivers.
1 is most likely relevant for other ACPI/SBIOS related hardware
components that rely on co-operating components doing what is expected
by the system design. If a device doesn't change it's behaviour in sync
with other devices when AC/DC changes, then bad (or at the very least
annoying) things may happen. When there is a SBIOS/ACPI/Driver stack in
place, the driver should honor the system design as well.
3 is only really relevant for the commercial side of things, but is a
real issue that takes up more engineering effort than the first 2 - my
cross to bear so to speak.
Regards,
Matthew
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