Re: [PATCH] drm/i915: Switch TGL-H DP-IN to dGFX when it's supported

[Karol Herbst]

On Thu, Aug 18, 2022 at 2:09 PM Lukas Wunner <lukas@xxxxxxxxx> wrote:
>
> On Tue, Aug 16, 2022 at 11:06:18AM +0300, Jani Nikula wrote:
> > On Tue, 16 Aug 2022, Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx> wrote:
> > > On mobile workstations like HP ZBook Fury G8, iGFX's DP-IN can switch to
> > > dGFX so external monitors are routed to dGFX, and more monitors can be
> > > supported as result.
> > >
> > > To switch the DP-IN to dGFX, the driver needs to invoke _DSM function 20
> > > on intel_dsm_guid2. This method is described in Intel document 632107.
> >
> > Is this the policy decision that we want to unconditionally make,
> > though?
>
> In general, we handle switching of outputs between GPUs in vga_switcheroo.c
> upon a request from user space via sysfs (well, debugfs currently).
> It's up to users to decide which policy suits their needs best.
>
> That said, we never grew support to allow different switching policies for
> the built-in panel and external outputs.  Laptops supporting this are
> rare.  Older MacBook Pros introduced between 2008 and 2010 are among them:
> They have separate muxes for the panel and external DP port.  Our policy
> is documented in a code comment in drivers/platform/x86/apple-gmux.c:
>
>  * The external DP port is only fully switchable on the first two unibody
>  * MacBook Pro generations, MBP5 2008/09 and MBP6 2010. This is done by an
>  * `NXP CBTL06141`_ which is controlled by gmux.
>  [...]
>  * Our switching policy for the external port is that on those generations
>  * which are able to switch it fully, the port is switched together with the
>  * panel when IGD / DIS commands are issued to vga_switcheroo. It is thus
>  * possible to drive e.g. a beamer on battery power with the integrated GPU.
>  * The user may manually switch to the discrete GPU if more performance is
>  * needed.
>  *
>  * On all newer generations, the external port can only be driven by the
>  * discrete GPU. If a display is plugged in while the panel is switched to
>  * the integrated GPU, *both* GPUs will be in use for maximum performance.
>  * To decrease power consumption, the user may manually switch to the
>  * discrete GPU, thereby suspending the integrated GPU.
>
> In other words, on these older MacBook Pros, we switch the panel and
> external DP port in unison, thus always allowing one of the two GPUs
> to runtime suspend and save power.
>
> Thanks,
>
> Lukas
>

sure, but this is changing now. I do have a laptop with a muxable
internal display. But this is considered to be a dynamic on demand
switching thing not a boot time switch.

Anyway, I am still not convinced that doing that unconditionally is
what we want, especially as userspace has to support dynamic switching
regardless.