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Date:   Wed, 18 Jul 2018 10:36:01 +0200
From:   Lukas Wunner <lukas@wunner.de>
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Lyude Paul <lyude@redhat.com>, nouveau@lists.freedesktop.org,
        David Airlie <airlied@linux.ie>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Ben Skeggs <bskeggs@redhat.com>,
        Linux PM <linux-pm@vger.kernel.org>
Subject: Re: [Nouveau] [PATCH 1/5] drm/nouveau: Prevent RPM callback
 recursion in suspend/resume paths
Message-ID: <20180718083601.GC16072@wunner.de>
References: <20180716235936.11268-1-lyude@redhat.com>
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On Wed, Jul 18, 2018 at 10:25:05AM +0200, Lukas Wunner wrote:
> The GPU contains an i2c subdevice for each connector with DDC lines.
> I believe those are modelled as children of the GPU's PCI device as
> they're accessed via mmio of the PCI device.
> 
> The problem here is that when the GPU's PCI device runtime suspends,
> its i2c child device needs to be runtime active to suspend the MST
> topology.  Catch-22.
> 
> I don't know whether or not it's necessary to suspend the MST topology.
> I'm not an expert on DisplayPort MultiStream transport.
> 
> BTW Lyude, in patch 4 and 5 of this series, you're runtime resuming
> pad->i2c->subdev.device->dev.  Is this the PCI device or is it the i2c
> device?  I'm always confused by nouveau's structs.  In nvkm_i2c_bus_ctor()
> I can see that the device you're runtime resuming is the parent of the
> i2c_adapter:
> 
> 	struct nvkm_device *device = pad->i2c->subdev.device;
> 	[...]
> 	bus->i2c.dev.parent = device->dev;
> 
> If the i2c_adapter is a child of the PCI device, it's sufficient
> to runtime resume the i2c_adapter, i.e. bus->i2c.dev, and this will
> implicitly runtime resume its parent.

Actually, having written all this I just remembered that we have this
in the documentation:

    8. "No-Callback" Devices

    Some "devices" are only logical sub-devices of their parent and cannot be
    power-managed on their own. [...]

    Subsystems can tell the PM core about these devices by calling
    pm_runtime_no_callbacks().

So it might actually be sufficient to just call pm_runtime_no_callbacks()
for the i2c devices...

Lukas