drm/displayid: add new displayid section/block iterators

Iterating DisplayID blocks across sections (in EDID extensions) is
unnecessarily complicated for the caller. Implement DisplayID iterators
to go through all blocks in all sections.

Usage example:

	const struct displayid_block *block;
	struct displayid_iter iter;

	displayid_iter_edid_begin(edid, &iter);
	displayid_iter_for_each(block, &iter) {
		/* operate on block */
	}
	displayid_iter_end(&iter);

When DisplayID is stored in EDID extensions, the DisplayID sections map
to extensions as described in VESA DisplayID v1.3 Appendix B: DisplayID
as an EDID Extension. This is implemented here.

When DisplayID is stored in its dedicated DDC device 0xA4, according to
VESA E-DDC v1.3, different rules apply for the structure. This is not
implemented here, as we don't currently use it, but the idea is you'd
have a different call for beginning the iteration, for example simply:

	displayid_iter_begin(displayid, &iter);

instead of displayid_iter_edid_begin(), and everything else would be
hidden away in the iterator functions.

v2:
- sizeof(struct displayid_block) -> sizeof(*block) (Ville)
- remove __ prefix from displayid_iter_block

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/da3dead1752ab16c061f7bd248ac1a4268f7fefb.1617024940.git.jani.nikula@intel.com

drivers/gpu/drm/drm_displayid.c

@@ -57,3 +57,77 @@ const u8 *drm_find_displayid_extension(const struct edid *edid,
 
 	return displayid;
 }
+
+void displayid_iter_edid_begin(const struct edid *edid,
+			       struct displayid_iter *iter)
+{
+	memset(iter, 0, sizeof(*iter));
+
+	iter->edid = edid;
+}
+
+static const struct displayid_block *
+displayid_iter_block(const struct displayid_iter *iter)
+{
+	const struct displayid_block *block;
+
+	if (!iter->section)
+		return NULL;
+
+	block = (const struct displayid_block *)&iter->section[iter->idx];
+
+	if (iter->idx + sizeof(*block) <= iter->length &&
+	    iter->idx + sizeof(*block) + block->num_bytes <= iter->length &&
+	    block->num_bytes > 0)
+		return block;
+
+	return NULL;
+}
+
+const struct displayid_block *
+__displayid_iter_next(struct displayid_iter *iter)
+{
+	const struct displayid_block *block;
+
+	if (!iter->edid)
+		return NULL;
+
+	if (iter->section) {
+		/* current block should always be valid */
+		block = displayid_iter_block(iter);
+		if (WARN_ON(!block)) {
+			iter->section = NULL;
+			iter->edid = NULL;
+			return NULL;
+		}
+
+		/* next block in section */
+		iter->idx += sizeof(*block) + block->num_bytes;
+
+		block = displayid_iter_block(iter);
+		if (block)
+			return block;
+	}
+
+	for (;;) {
+		iter->section = drm_find_displayid_extension(iter->edid,
+							     &iter->length,
+							     &iter->idx,
+							     &iter->ext_index);
+		if (!iter->section) {
+			iter->edid = NULL;
+			return NULL;
+		}
+
+		iter->idx += sizeof(struct displayid_hdr);
+
+		block = displayid_iter_block(iter);
+		if (block)
+			return block;
+	}
+}
+
+void displayid_iter_end(struct displayid_iter *iter)
+{
+	memset(iter, 0, sizeof(*iter));
+}

include/drm/drm_displayid.h

@@ -108,4 +108,22 @@ const u8 *drm_find_displayid_extension(const struct edid *edid,
 				       int *length, int *idx,
 				       int *ext_index);
 
+/* DisplayID iteration */
+struct displayid_iter {
+	const struct edid *edid;
+
+	const u8 *section;
+	int length;
+	int idx;
+	int ext_index;
+};
+
+void displayid_iter_edid_begin(const struct edid *edid,
+			       struct displayid_iter *iter);
+const struct displayid_block *
+__displayid_iter_next(struct displayid_iter *iter);
+#define displayid_iter_for_each(__block, __iter) \
+	while (((__block) = __displayid_iter_next(__iter)))
+void displayid_iter_end(struct displayid_iter *iter);
+
 #endif