FF_DOP_CLOCK_GATE_DISABLE);
}
- if (IS_GRAPHICS_VER(i915, 9, 12)) {
- /* FtrPerCtxtPreemptionGranularityControl:skl,bxt,kbl,cfl,cnl,icl,tgl */
+ /*
+ * Intel platforms that support fine-grained preemption (i.e., gen9 and
+ * beyond) allow the kernel-mode driver to choose between two different
+ * options for controlling preemption granularity and behavior.
+ *
+ * Option 1 (hardware default):
+ * Preemption settings are controlled in a global manner via
+ * kernel-only register CS_DEBUG_MODE1 (0x20EC). Any granularity
+ * and settings chosen by the kernel-mode driver will apply to all
+ * userspace clients.
+ *
+ * Option 2:
+ * Preemption settings are controlled on a per-context basis via
+ * register CS_CHICKEN1 (0x2580). CS_CHICKEN1 is saved/restored on
+ * context switch and is writable by userspace (e.g., via
+ * MI_LOAD_REGISTER_IMMEDIATE instructions placed in a batch buffer)
+ * which allows different userspace drivers/clients to select
+ * different settings, or to change those settings on the fly in
+ * response to runtime needs. This option was known by name
+ * "FtrPerCtxtPreemptionGranularityControl" at one time, although
+ * that name is somewhat misleading as other non-granularity
+ * preemption settings are also impacted by this decision.
+ *
+ * On Linux, our policy has always been to let userspace drivers
+ * control preemption granularity/settings (Option 2). This was
+ * originally mandatory on gen9 to prevent ABI breakage (old gen9
+ * userspace developed before object-level preemption was enabled would
+ * not behave well if i915 were to go with Option 1 and enable that
+ * preemption in a global manner). On gen9 each context would have
+ * object-level preemption disabled by default (see
+ * WaDisable3DMidCmdPreemption in gen9_ctx_workarounds_init), but
+ * userspace drivers could opt-in to object-level preemption as they
+ * saw fit. For post-gen9 platforms, we continue to utilize Option 2;
+ * even though it is no longer necessary for ABI compatibility when
+ * enabling a new platform, it does ensure that userspace will be able
+ * to implement any workarounds that show up requiring temporary
+ * adjustments to preemption behavior at runtime.
+ *
+ * Notes/Workarounds:
+ * - Wa_14015141709: On DG2 and early steppings of MTL,
+ * CS_CHICKEN1[0] does not disable object-level preemption as
+ * it is supposed to (nor does CS_DEBUG_MODE1[0] if we had been
+ * using Option 1). Effectively this means userspace is unable
+ * to disable object-level preemption on these platforms/steppings
+ * despite the setting here.
+ *
+ * - Wa_16013994831: May require that userspace program
+ * CS_CHICKEN1[10] when certain runtime conditions are true.
+ * Userspace requires Option 2 to be in effect for their update of
+ * CS_CHICKEN1[10] to be effective.
+ *
+ * Other workarounds may appear in the future that will also require
+ * Option 2 behavior to allow proper userspace implementation.
+ */
+ if (GRAPHICS_VER(i915) >= 9)
wa_masked_en(wal,
GEN7_FF_SLICE_CS_CHICKEN1,
GEN9_FFSC_PERCTX_PREEMPT_CTRL);
- }
if (IS_SKYLAKE(i915) ||
IS_KABYLAKE(i915) ||