drm/i915/gt: Document function to decode register state context

It's not obvious how the encode/decode of the per platform tables is
done. Document it so while adding tables for new platforms people can be
confident they right things is being done.
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220930050903.3479619-3-lucas.demarchi@intel.com

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -20,6 +20,30 @@
 #include "intel_ring.h"
 #include "shmem_utils.h"
 
+/*
+ * The per-platform tables are u8-encoded in @data. Decode @data and set the
+ * addresses' offset and commands in @regs. The following encoding is used
+ * for each byte. There are 2 steps: decoding commands and decoding addresses.
+ *
+ * Commands:
+ * [7]: create NOPs - number of NOPs are set in lower bits
+ * [6]: When creating MI_LOAD_REGISTER_IMM command, allow to set
+ *      MI_LRI_FORCE_POSTED
+ * [5:0]: Number of NOPs or registers to set values to in case of
+ *        MI_LOAD_REGISTER_IMM
+ *
+ * Addresses: these are decoded after a MI_LOAD_REGISTER_IMM command by "count"
+ * number of registers. They are set by using the REG/REG16 macros: the former
+ * is used for offsets smaller than 0x200 while the latter is for values bigger
+ * than that. Those macros already set all the bits documented below correctly:
+ *
+ * [7]: When a register offset needs more than 6 bits, use additional bytes, to
+ *      follow, for the lower bits
+ * [6:0]: Register offset, without considering the engine base.
+ *
+ * This function only tweaks the commands and register offsets. Values are not
+ * filled out.
+ */
 static void set_offsets(u32 *regs,
 			const u8 *data,
 			const struct intel_engine_cs *engine,