[PATCH] drm/cma-helper: Set VM_DONTEXPAND for mmap

[Alyssa Rosenzweig]

> drm_gem_cma_mmap() cannot assume every implementation of dma_mmap_wc()
> will end up calling remap_pfn_range() (which happens to set the relevant
> vma flag, among others), so in order to make sure expectations around
> VM_DONTEXPAND are met, let it explicitly set the flag like most other
> GEM mmap implementations do.
> 
> This avoids repeated warnings on a small minority of systems where the
> display is behind an IOMMU, and has a simple driver which does not
> override drm_gem_cma_default_funcs.

Apple system-on-chips have their display behind an IOMMU. Actually, a
separate IOMMU for each display, and a separate IOMMU for each display
controller -- so there are 4 IOMMUs total for display on the M1.

I've tested this patch against my work-in-progress display driver for
the M1. It indeed fixes the annoying warnings every frame (wayland) and
on mode setting (x11). So this is

	Tested-by: Alyssa Rosenzweig <alyssa at rosenzweig.io>

I've cherry-picked the patch into my M1 staging/downstream tree, so I
guess that's an Acked-by. I don't know anything about the vm_* stuff in
the kernel yet, though, since can't give a reviewed-by. Will leave that
one to the pro's.

I know you were trying to fix an HDLCD issue, but I needed this patch
too, so thank you! 😄