Question regarding forcewake in i915

From: 유재용 <jaeyong.yoo@samsung.com>
To: intel-gfx@lists.freedesktop.org
Subject: Question regarding forcewake in i915
Date: Mon, 22 Dec 2014 12:26:24 +0000 (GMT)	[thread overview]
Message-ID: <507709752.66831419251184872.JavaMail.weblogic@epmlwas06a> (raw)

Hello intel-gfx,
I'm reading i915 gpu drivers and find myself quite hard to understand about forcewake concepts.
I understand that it is something with the energy efficiency so related to ACPI. And it looks like forcewake is working as a pair (get and put).
In the "get" part, what it first does it waiting on FORCEWAKE_ACK_HSW register (in case of haswell).
And then, it writes something to FORCEWAKE_MT register, read from ECOBUS.
And again, it waits on FORCEWAKE_ACK_HSW again!
It becomes more confusing when it comes to put.
In the "put" part, what it does it writing to FORCEWAKE_MT register and read from ECOBUS.

I tried to find some good reading materials about this forcewake, but what I found was a series of patches in this mailing list. (which are quite hard to follow from the begining)
Could you explain about the concept of FORCEWAKE and possibly the magic tricks on these get and put?

Thanks,
Jaeyong

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