drm: simple_kms_helper: Fix .mode_valid() documentation

This fixes up the .mode_valid() vtable entry documentation
by copyediting the documentation from the .mode_valid()
documentation in the drm_modeset_helper_vtables.h file.

Fixes: 40275dc4 ("drm: simple_kms_helper: Add mode_valid() callback support")
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227101109.6088-1-linus.walleij@linaro.org

include/drm/drm_simple_kms_helper.h

@@ -24,9 +24,30 @@ struct drm_simple_display_pipe_funcs {
 	/**
 	 * @mode_valid:
 	 *
-	 * This function is called to filter out valid modes from the
-	 * suggestions suggested by the bridge or display. This optional
-	 * hook is passed in when initializing the pipeline.
+	 * This callback is used to check if a specific mode is valid in the
+	 * crtc used in this simple display pipe. This should be implemented
+	 * if the display pipe has some sort of restriction in the modes
+	 * it can display. For example, a given display pipe may be responsible
+	 * to set a clock value. If the clock can not produce all the values
+	 * for the available modes then this callback can be used to restrict
+	 * the number of modes to only the ones that can be displayed. Another
+	 * reason can be bandwidth mitigation: the memory port on the display
+	 * controller can have bandwidth limitations not allowing pixel data
+	 * to be fetched at any rate.
+	 *
+	 * This hook is used by the probe helpers to filter the mode list in
+	 * drm_helper_probe_single_connector_modes(), and it is used by the
+	 * atomic helpers to validate modes supplied by userspace in
+	 * drm_atomic_helper_check_modeset().
+	 *
+	 * This function is optional.
+	 *
+	 * NOTE:
+	 *
+	 * Since this function is both called from the check phase of an atomic
+	 * commit, and the mode validation in the probe paths it is not allowed
+	 * to look at anything else but the passed-in mode, and validate it
+	 * against configuration-invariant hardware constraints.
 	 *
 	 * RETURNS:
 	 *