[Bug 108850] i915 "kernel NULL pointer dereference" in kernel-4.18.20

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108850

--- Comment #1 from Chris Wilson <chris at chris-wilson.co.uk> ---
Missing backport of

commit d9d117e40d4ffc03438177eeac83d96dfeee76be
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Mon Jun 11 18:18:25 2018 +0100

    drm/i915/ringbuffer: Serialize load of PD_DIR

    After triggering the mm switch with a load of PD_DIR, which may be
    deferred unto the MI_SET_CONTEXT on rcs, serialise the next commands
    with that load by posting a read of PD_DIR (or else those subsequent
    commands may access the stale page tables).

    Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
    Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
    Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
    Cc: Matthew Auld <matthew.william.auld at gmail.com>
    Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
    Link:
https://patchwork.freedesktop.org/patch/msgid/20180611171825.13678-2-c
hris at chris-wilson.co.uk

or at least the active ingredient of always allocating the engine->scratch.

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