Integrating graphics switching into refind

[Mohammed Kabir]

Hi Rod,

Long-time refind user here! Thanks for the absolutely awesome bootloader.

I recently got a MacBook Pro 2016 (MBP 13,3) with 2 GPUs (the integrated Intel Iris Pro and the discrete AMD RX 460). I'm currently working on getting Linux running smoothly on this model, and I ran into a couple of issues.

We are tracking the general progress on Linux here : https://github.com/Dunedan/mbp-2016-linux

Specifically, I'm trying to get Linux to use the integrated intel graphics card (very interesting details here https://github.com/Dunedan/mbp-2016-linux/issues/6). Now I have the thing working using a utility called gpu-switch (https://github.com/0xbb/gpu-switch/) which is doing some EFI magic to switch cards. Refind's "spoof_osx_version 10.12" is used to power on the card.

Now the problem is, if I run the utilty from Linux and set it to integrated graphics and reboot, I can get Linux running perfectly on the Intel GPU. However, when I reboot into macOS, boot hangs. This seems to be connected to gpu-switch, since if I go back into Linux and switch it to the discrete graphics, macOS boots fine. I therefore can't seamlessly reboot into macOS or Linux simultaneously. This leads me to think that we can streamline things a lot if we can get refind to do some automatic stuff here :
1. If we are booting into macOS, set the dGPU option.
2. If we are booting into Linux, set the iGPU option.

Do you think this is possible to do? The gpu-switch source should be able to give you implementation details on what's needed.

Thanks!

[Roderick W. Smith]

It looks like the script you reference is just writing one value to the gpu-power-prefs EFI NVRAM variable to enable one device, and another value to enable another device. That part would be easy enough to implement in rEFInd; it already modifies the csr-active-config variable, the PreviousBoot variable, and one or two others under certain circumstances, so the code for writing to gpu-power-prefs would be similar.

The tricky part is in figuring out what to write and under what circumstances. That detail is likely to be very hardware- and OS-specific -- that is, you might need to write 1 for Windows and Linux but 0 for macOS on Machine A, but 1 for Windows and MacOS and 0 for Linux on Machine B. Then there's the question of what to do with more obscure OSes, BIOS-mode boots, etc. Because all of this would likely vary from one machine to another, you'd probably need a configuration file option to tell rEFInd what to write for particular OSes. That task could be complex enough to require some new option parsing code. Alternatively, a new configuration file in the same directory as the boot loader (akin to refind_linux.conf) could be used to set the option on a per-OS basis, but that adds its own complexity.

In any event, a crude proof-of-concept version that sets the values in a hard-coded way for your own computer would be the best first step. I may be able to get to that in a few days, but if you want to give it a try yourself, you'd need to use the EfivarSetRaw() function in refind/lib.c to write the variable. You'd need to set the 0 or 1 values you're doing via your Bash script, perhaps from StartEFIImage() or StartEFIImageList() in refind/main.c, keying off of the OSType variable to set the appropriate value. If the desired behavior can be created in a hard-coded way, then we can start to consider the user interface details; but if it doesn't work from rEFInd for some reason, it's probably best to leave the user interface issues until the hard-coded switching can be made to work.