drm/displayid: add separate drm_displayid.c

We'll be adding more DisplayID specific functions going forward, so
start off by splitting out a few functions to a separate file.

We don't bother with exporting the functions; at least for now they
should be needed solely within drm.ko.

No functional changes.

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/07942d5011891b8e8f77245c78b34f4af97a9315.1617024940.git.jani.nikula@intel.com

1 files changed, 59 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_displayid.c b/drivers/gpu/drm/drm_displayid.c
new file mode 100644
index 000000000000..908bbe6feb61
--- /dev/null
+++ b/drivers/gpu/drm/drm_displayid.c
@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: MIT
+/*
+ * Copyright © 2021 Intel Corporation
+ */
+
+#include <drm/drm_displayid.h>
+#include <drm/drm_edid.h>
+#include <drm/drm_print.h>
+
+static int validate_displayid(const u8 *displayid, int length, int idx)
+{
+	int i, dispid_length;
+	u8 csum = 0;
+	const struct displayid_hdr *base;
+
+	base = (const struct displayid_hdr *)&displayid[idx];
+
+	DRM_DEBUG_KMS("base revision 0x%x, length %d, %d %d\n",
+		      base->rev, base->bytes, base->prod_id, base->ext_count);
+
+	/* +1 for DispID checksum */
+	dispid_length = sizeof(*base) + base->bytes + 1;
+	if (dispid_length > length - idx)
+		return -EINVAL;
+
+	for (i = 0; i < dispid_length; i++)
+		csum += displayid[idx + i];
+	if (csum) {
+		DRM_NOTE("DisplayID checksum invalid, remainder is %d\n", csum);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+const u8 *drm_find_displayid_extension(const struct edid *edid,
+				       int *length, int *idx,
+				       int *ext_index)
+{
+	const u8 *displayid = drm_find_edid_extension(edid, DISPLAYID_EXT, ext_index);
+	const struct displayid_hdr *base;
+	int ret;
+
+	if (!displayid)
+		return NULL;
+
+	/* EDID extensions block checksum isn't for us */
+	*length = EDID_LENGTH - 1;
+	*idx = 1;
+
+	ret = validate_displayid(displayid, *length, *idx);
+	if (ret)
+		return NULL;
+
+	base = (const struct displayid_hdr *)&displayid[*idx];
+	*length = *idx + sizeof(*base) + base->bytes;
+
+	return displayid;
+}