When DMAR / VT-d is enabled, the display engine uses overfetching,
presumably to deal with the increased latency. To avoid display engine
errors and DMAR faults, as a workaround the GGTT is populated with scatch
PTEs when VT-d is enabled. However starting with gen10, Write-combined
writing of scratch PTES is no longer possible and as a result, populating
the full GGTT with scratch PTEs like on resume becomes very slow as
uncached access is needed.

Therefore, on integrated GPUs utilize the fact that the PTEs are stored in
stolen memory which retain content across S3 suspend. Don't clear the PTEs
on suspend and resume. This improves on resume time with around 100 ms.
While 100+ms might appear like a short time it's 10% to 20% of total resume
time and important in some applications.

One notable exception is Intel Rapid Start Technology which may cause
stolen memory to be lost across what the OS percieves as S3 suspend.
If IRST is enabled or if we can't detect whether IRST is enabled, retain
the old workaround, clearing and re-instating PTEs.

As an additional measure, if we detect that the last ggtt pte was lost
during suspend, print a warning and re-populate the GGTT ptes

On discrete GPUs, the display engine scans out from LMEM which isn't
subject to DMAR, and presumably the workaround is therefore not needed,
but that needs to be verified and disabling the workaround for dGPU,
if possible, will be deferred to a follow-up patch.

v2:
- Rely on retained ptes to also speed up suspend and resume re-binding.
- Re-build GGTT ptes if Intel rst is enabled.
v3:
- Re-build GGTT ptes also if we can't detect whether Intel rst is enabled,
  and if the guard page PTE and end of GGTT was lost.
v4:
- Fix some kerneldoc issues (Matthew Auld), rebase.
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220617152856.249295-1-thomas.hellstrom@linux.intel.com