drm/i915: fix comment on I915_{READ, WRITE}_FW

Comment mentioned use of intel_uncore_forcewake_irq{unlock, lock}
functions which are nonexistent (and never were).

The description was also incomplete and could cause confusion. Updated
comment is more elaborate on usage and caveats.

v2: mention __locked variant of intel_uncore_forcewake_{get,put} instead
    of plain ones

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilsono.c.uk>
[Mika: removed two superfluous lines on comment noted by Chris]
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477399682-3133-1-git-send-email-arkadiusz.hiler@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -3844,11 +3844,30 @@ __raw_write(64, q)
 #undef __raw_write
 
 /* These are untraced mmio-accessors that are only valid to be used inside
- * critical sections inside IRQ handlers where forcewake is explicitly
+ * critical sections, such as inside IRQ handlers, where forcewake is explicitly
  * controlled.
+ *
  * Think twice, and think again, before using these.
- * Note: Should only be used between intel_uncore_forcewake_irqlock() and
- * intel_uncore_forcewake_irqunlock().
+ *
+ * As an example, these accessors can possibly be used between:
+ *
+ * spin_lock_irq(&dev_priv->uncore.lock);
+ * intel_uncore_forcewake_get__locked();
+ *
+ * and
+ *
+ * intel_uncore_forcewake_put__locked();
+ * spin_unlock_irq(&dev_priv->uncore.lock);
+ *
+ *
+ * Note: some registers may not need forcewake held, so
+ * intel_uncore_forcewake_{get,put} can be omitted, see
+ * intel_uncore_forcewake_for_reg().
+ *
+ * Certain architectures will die if the same cacheline is concurrently accessed
+ * by different clients (e.g. on Ivybridge). Access to registers should
+ * therefore generally be serialised, by either the dev_priv->uncore.lock or
+ * a more localised lock guarding all access to that bank of registers.
  */
 #define I915_READ_FW(reg__) __raw_i915_read32(dev_priv, (reg__))
 #define I915_WRITE_FW(reg__, val__) __raw_i915_write32(dev_priv, (reg__), (val__))