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.
