drm/i915: Fixup intel_write_status_page() for old CPUs without clflush

Not all of our target platforms have clflush. For those without, just
assume the status page is sufficiently coherent that we do not need our
paranoia.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: 14a6bbf9e535 ("drm/i915: Replace irq_seqno_barrier on hws write with a clflush")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170324163540.31981-1-chris@chris-wilson.co.uk
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -454,14 +454,22 @@ intel_read_status_page(struct intel_engine_cs *engine, int reg)
 }
 
 static inline void
-intel_write_status_page(struct intel_engine_cs *engine,
-			int reg, u32 value)
+intel_write_status_page(struct intel_engine_cs *engine, int reg, u32 value)
 {
-	mb();
-	clflush(&engine->status_page.page_addr[reg]);
-	engine->status_page.page_addr[reg] = value;
-	clflush(&engine->status_page.page_addr[reg]);
-	mb();
+	/* Writing into the status page should be done sparingly. Since
+	 * we do when we are uncertain of the device state, we take a bit
+	 * of extra paranoia to try and ensure that the HWS takes the value
+	 * we give and that it doesn't end up trapped inside the CPU!
+	 */
+	if (static_cpu_has(X86_FEATURE_CLFLUSH)) {
+		mb();
+		clflush(&engine->status_page.page_addr[reg]);
+		engine->status_page.page_addr[reg] = value;
+		clflush(&engine->status_page.page_addr[reg]);
+		mb();
+	} else {
+		WRITE_ONCE(engine->status_page.page_addr[reg], value);
+	}
 }
 
 /*