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It turns out that this code was previously dead.

c/s dcf41790

 " x86/mmcfg/drhd: Move acpi_mmcfg_init() call before calling
acpi_parse_dmar()" resulted in PCI segment 0 now having been initialised
enough for acpi_parse_one_drhd() to not take the

  /* Skip checking if segment is not accessible yet. */

path unconditionally.  However, some systems have DMAR tables which list
devices which are disabled by user choice (in particular, Dell PowerEdge R740
with I/O AT DMA disabled), and turning off all IOMMU functionality in this
case is entirely unhelpful behaviour.

Leave the warning which identifies the problematic devices, but drop the
remaining logic.  This leaves the system in better overall state, and working
in the same way that it did in previous releases.
Reported-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>