linux-kernel - Re: [PATCH v1 2/6] dt-bindings: drm/msm/a6xx: Add GX power-domain for GMU bindings

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Date:   Tue, 19 Feb 2019 09:12:19 -0700
From:   Jordan Crouse <jcrouse@...eaurora.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        freedreno <freedreno@...ts.freedesktop.org>, sboyd@...nel.org,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        Mark Rutland <mark.rutland@....com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v1 2/6] dt-bindings: drm/msm/a6xx: Add GX power-domain
 for GMU bindings

On Sun, Feb 17, 2019 at 05:43:16PM -0500, Rob Clark wrote:
> On Sun, Feb 17, 2019 at 4:08 PM Rob Herring <robh+dt@...nel.org> wrote:
> >
> > On Mon, Feb 4, 2019 at 10:15 AM Jordan Crouse <jcrouse@...eaurora.org> wrote:
> > >
> > > The GMU should have two power domains defined: "cx" and "gx". "cx" is the
> > > actual power domain for the device and "gx" will be attached at runtime
> > > to manage reference counting on the GPU device in case of a GMU crash.
> >
> > power-domains are supposed to be actual regions on a chip die which
> > can be power gated. However, they are often abused by being defined in
> > terms of kernel PM domains which are not always the same thing. This
> > description sounds like the latter case.
> >
> 
> iirc (and Jordan can correct me), this arrangement was needed because
> normally the GMU does the GPU power control (except for if we manage
> to crash it and need to reset the GMU)..
> 
> so maybe not 100% about the actual regions on chip die which can be
> gated.. but it is a reality of how hw + fw + sw fit together..

Ack - forgot to add Stephen who knows about this.

Rob is correct. The GX domain is real but it is normally controlled by an
off-CPU microcontroller. The CPU needs to get involved only when the
microcontroller crashes and we need to get things back to normal.  So the
description of it being an actual region on a chip die is accurate.

It sounds like I need to describe the hardware better in the bindings document.

Jordan

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