Re: [Intel-gfx] [Nouveau] [PATCH 0/5] Fix deadlock on runtime suspend in DRM drivers

[Alex Deucher]

On Mon, Feb 19, 2018 at 9:54 AM, Daniel Vetter <daniel@xxxxxxxx> wrote:
> On Mon, Feb 19, 2018 at 03:47:42PM +0100, Lukas Wunner wrote:
>> On Mon, Feb 19, 2018 at 03:05:53PM +0100, Daniel Vetter wrote:
>> > On Mon, Feb 19, 2018 at 12:58:17PM +0100, Lukas Wunner wrote:
>> > > On Mon, Feb 19, 2018 at 12:34:43PM +0100, Daniel Vetter wrote:
>> > > > Well, userspace expects hotplug events, even when we runtime suspend
>> > > > stuff. Hence waking shit up with polling. Imo ignoring hotplug events is a
>> > > > pretty serious policy decision which is ok in the context of
>> > > > vga_switcheroo, but not really as an automatic thing. E.g. usb also wakes
>> > > > up if you plug something in, even with all the runtime pm stuff enabled.
>> > > > Same for sata and everything else.
>> > >
>> > > On the MacBook Pro, the HPD pin of external DP and HDMI ports goes into
>> > > the gmux controller, which sends an interrupt on hotplug even if the GPU
>> > > is powered down.
>> > >
>> > > Apparently Optimus has a similar functionality, cf. 3a6536c51d5d.
>> >
>> > Yeah, for vga_switcheroo and gmux setups shutting down polling explicitly
>> > makes sense. I think ideally we'd stop polling in the gmux handler somehow
>> > (maybe by dropping the relevant DRM_CONNECTOR_POLL flags, or outright
>> > stopping it all). But not when runtime suspending the entire gpu (e.g.
>> > idle system that shuts down the screen and everything, before it decides
>> > a few minutes later to do a full system suspend).
>>
>> nouveau, radeon and amdgpu currently use runtime PM *only* on hybrid
>> graphics laptops.
>>
>> Should the drivers later be extended to also use runtime PM in other
>> scenarios (desktop machines, eGPUs), they can easily detect whether
>> to disable polling on runtime suspend by calling apple_gmux_present()
>> on Macs or the equivalent for Optimus/ATPX.
>
> Ah, then I think the current solution is ok (if not entirely clean imo,
> but that can be fixed up whenever it hurts). Implementing runtime pm for
> other cases is up to the driver authors really (probably more pressing
> when the gpu is on the same SoC).

On our APUs, we support fairly fine grained powergating so this mostly
happens auto-magically in hw; no need for runtimepm.  We haven't
supported native analog encoders in last 3 or 4 generations of display
hw, so polling is not much of an issue going forward.  On most
integrated platforms (e.g., laptops and all-in-ones), digital hotplug
is handled by the platform (we get an ACPI ATIF notification) so we
can wake the dGPU.

Alex

> -Daniel
>
>>
>> Thanks,
>>
>> Lukas
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>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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