Enable Message Signaled Interrupts (MSI)

[valpackett]

This fixes ridiculous interrupt rate on aarch64, was enabled in the legacy in-tree drm port, and is a good idea in general.

Corresponding kernel patch: https://reviews.freebsd.org/D21008 but this shouldn't break the build without it, thanks to if defined(pci_enable_msi).

Tested on amdgpu and i915kms.

[hselasky]

@johalun , @zeising --- This patch needs to go to FreeBSD-current and FreeBSD-12 DRM next before we can merge the kernel part! Will you handle.

[zeising]

Just out of curiousity, where are pci_[enable,disable]_msi defined, and for which architecutures?

[hselasky]

They are currently not defined. Looks like a leftover from a porting effort.

[zeising]

Then how is this supposed to work? It's just a change to code that was
#ifdef __linux__ is now #if defined(__linux__) || defined (pci_[enable,disable]_msi
Or is there an upcoming change to define pci_[enable,disable]_msi?

[valpackett]

The kernel patch is linked in the first post

[zeising]

@myfreeweb That's probably the piece I'm missing. Sorry for the confusion.
@myfreeweb @hselasky Thanks for explaining to me. :)

[zeising]

I'd like to give @johalun the chance to comment, but I see no problems with this.

[johalun]

Has this been tested on other than aarch64+amd?

[valpackett]

Of course I've tested on a Ryzen+Vega desktop and an i915 laptop

[johalun]

@hselasky Also merged to v4.16 and v4.16-fbsd12