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https://bugs.launchpad.net/bugs/1769843

Title:
  [i915 CNL-Y] system hangs soon after bootup

Status in linux-oem package in Ubuntu:
  Fix Released
Status in linux-oem source package in Bionic:
  Fix Released

Bug description:
  CNL-Y SDP machine hangs soon after logging in, and needs this from
  intel-gfx list:

  Author: Chris Wilson <ch...@chris-wilson.co.uk>
  Date:   Tue May 8 13:15:00 2018 +0300

      drm/i915/execlists: Use rmb() to order CSB reads
      
      We assume that the CSB is written using the normal ringbuffer
      coherency protocols, as outlined in kernel/events/ring_buffer.c:
      
          *   (HW)                              (DRIVER)
          *
          *   if (LOAD ->data_tail) {            LOAD ->data_head
          *                      (A)             smp_rmb()       (C)
          *      STORE $data                     LOAD $data
          *      smp_wmb()       (B)             smp_mb()        (D)
          *      STORE ->data_head               STORE ->data_tail
          *   }
      
      So we assume that the HW fulfils it's ordering requirements, and so we
      should use a complimentary rmb() to ensure that our read of its WRITE
      pointer is completed before we start accessing the data.
      
      The final mb() is implied by the uncached mmio we perform to inform the
      HW of our READ pointer.
      
      References: https://bugs.freedesktop.org/show_bug.cgi?id=105064
      References: https://bugs.freedesktop.org/show_bug.cgi?id=105888
      References: https://bugs.freedesktop.org/show_bug.cgi?id=106185
      References: 61bf9719fa17 ("drm/i915/cnl: Use mmio access to context 
status buffer")
      Suggested-by: Mika Kuoppala <mika.kuopp...@linux.intel.com>
      Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
      Cc: Mika Kuoppala <mika.kuopp...@linux.intel.com>
      Cc: Joonas Lahtinen <joonas.lahti...@linux.intel.com>
      Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
      Cc: MichaƂ Winiarski <michal.winiar...@intel.com>
      Cc: Rafael Antognolli <rafael.antogno...@intel.com>
      Cc: Michel Thierry <michel.thie...@intel.com>
      Cc: Timo Aaltonen <tjaal...@ubuntu.com>
      Signed-off-by: Timo Aaltonen <timo.aalto...@canonical.com>

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