On Sun, Jul 10, 2016 at 03:20:13PM +0200, Lukas Wunner wrote: > Hi Peter, > > > [12:42] Lekensteyn: Should the video card always be powered up when a > > register is read from the HDMI audio controller? Or would it be > > better to leave the video card suspended and just fail the HDA > > register reads? > > It should probably be powered up. > > > > [12:43] Lekensteyn: I'm trying to figure out how vga_switcheroo should > handle this, maybe l1k has an idea? > > [12:48] Lekensteyn: The current implemented policy is that the video card > > dictates the power state and that the HDMI audio device has to > > adapt (even if it is seemingly in use) > > This current architecture seems to have come about historically (Dave > Airlie first implemented vga_switcheroo with manual power control, > then added runtime pm in a second step). > > It may also be motivated by the fact that the driver core is currently > not supporting dependencies between devices beyond the hierarchical > parent/child relationship. > > Rafael Wysocki (cc:) posted a series in January addressing the latter > problem with so-called "device links". The series was reposted recently > by Marek Szyprowski: > https://www.mail-archive.com/linux-kernel at vger.kernel.org/msg1170031.html > > The vga_switcheroo audio handling should probably be reworked to use such > "device links". The result would be that the GPU won't runtime suspend > as long as a ref is held for the audio card. The audio card needs to then > make sure that it releases any refs if it has nothing to do. > > The "device links" series seems to impose more restrictions than we > actually need here, it seems to require that the "supplier" is bound > to a driver before the "consumer" may probe. IOW nouveau needs to be > bound before snd_hda_audio can probe. I'm not sure if that additional > (unnecessary) restriction is a problem.
The above (including the load ordering restriction, which we resolve using EPROBE_DEFER) is how we hand-rolled a fix for this between i915 and snd-hda-intel. Not for vga-switcheroo, but for rpm depencies between gpu and snd in general. Not sure why exactly vga-switcheroo jumps into this here ... but sounds like the device links stuff is the way to go. > I've recently tried to add runtime pm for muxed laptops to vga_switcheroo, > this is something that Pierre Moreau (cc:) has expressed an interest in > for his MacBook Pro. I came across a major roadblock in the form of > vga_switcheroo_set_dynamic_switch(). That function is called from the > DRM driver when the GPU runtime suspends and resumes. It takes the > vgasr_mutex. The problem is, if the user initiates a switch of the mux, > that mutex is already taken in vga_switcheroo_debugfs_write(). If the > GPU we're switching to is asleep, it'll wake up for the switch and > we'll get a deadlock when the DRM driver subsequently calls > vga_switcheroo_set_dynamic_switch(). > > I would like to get rid of vga_switcheroo_set_dynamic_switch() to solve > this. The function has two purposes: Number one, vga_switcheroo updates > its internal power state representation for the GPU. That is actually > pointless for driver power control because we can always query the > driver core for this information (e.g. pm_runtime_suspended()). > The second purpose is to switch the audio client on and off. If we would > use a "device link" to express the dependency between the audio device > and the GPU, we wouldn't need this. So moving to "device links" is > a prerequisite to make runtime pm work for muxed laptops. right, expressing the depencies explicitly (and managing it on the audio side) would also solve this deadlock. > If you want to take a stab at using "device links" for vga_switcheroo > then please go ahead as I'm swamped with other work. (I've reverse- > engineered retrieval of Apple device properties from EFI this week, > which is needed for Thunderbolt.) Let me know if you have any questions > or need stuff reviewed or whatever. Since the "device links" series > hasn't landed yet, it's still possible I think to ask for feature > requests should it not work for this particular use case. The > linux-pm at vger.kernel.org mailing list is the right place to inquire > about the series. > > Thanks for raising this important question. +1 from me on the overall plan. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
