drm/amd/display: add explanation for surface_update_type

void dc_update_surfaces_for_stream(struct dc *dc, struct dc_surface_update *updates,

		int surface_count, const struct dc_stream *stream);

/* Surface update type is used by dc_update_surfaces_and_stream

 * The update type is determined at the very beginning of the function based

 * on parameters passed in and decides how much programming (or updating) is

 * going to be done during the call.

 *

 * UPDATE_TYPE_FAST is used for really fast updates that do not require much

 * logical calculations or hardware register programming. This update MUST be

 * ISR safe on windows. Currently fast update will only be used to flip surface

 * address.

 *

 * UPDATE_TYPE_MED is used for slower updates which require significant hw

 * re-programming however do not affect bandwidth consumption or clock

 * requirements. At present, this is the level at which front end updates

 * that do not require us to run bw_calcs happen. These are in/out transfer func

 * updates, viewport offset changes, recout size changes and pixel depth changes.

 * This update can be done at ISR, but we want to minimize how often this happens.

 *

 * UPDATE_TYPE_FULL is slow. Really slow. This requires us to recalculate our

 * bandwidth and clocks, possibly rearrange some pipes and reprogram anything front

 * end related. Any time viewport dimensions, recout dimensions, scaling ratios or

 * gamma need to be adjusted or pipe needs to be turned on (or disconnected) we do

 * a full update. This cannot be done at ISR level and should be a rare event.

 * Unless someone is stress testing mpo enter/exit, playing with colour or adjusting

 * underscan we don't expect to see this call at all.

 */

enum surface_update_type {

	UPDATE_TYPE_FAST, /* super fast, safe to execute in isr */

	UPDATE_TYPE_MED,  /* a lot of programming needed.  may need to alloc */

	UPDATE_TYPE_MED,  /* ISR safe, most of programming needed, no bw/clk change*/

	UPDATE_TYPE_FULL, /* may need to shuffle resources */

};