drm/edid: Allow querying/working with the panel ID from the EDID

EDIDs have 32-bits worth of data which is intended to be used to
uniquely identify the make/model of a panel. This has historically
been used only internally in the EDID processing code to identify
quirks with panels.

We'd like to use this panel ID in panel drivers to identify which
panel is hooked up and from that information figure out power sequence
timings. Let's expose this information from the EDID code and also
allow it to be accessed early, before a connector has been created.

To make matching in the panel drivers code easier, we'll return the
panel ID as a 32-bit value. We'll provide some functions for
converting this value back and forth to something more human readable.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210914132020.v5.3.I4a672175ba1894294d91d3dbd51da11a8239cf4a@changeid

1 files changed, 67 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 520fe1391769..f84e0dd264f4 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -2087,6 +2087,73 @@ struct edid *drm_get_edid(struct drm_connector *connector,
 }
 EXPORT_SYMBOL(drm_get_edid);
 
+static u32 edid_extract_panel_id(const struct edid *edid)
+{
+	/*
+	 * In theory we could try to de-obfuscate this like edid_get_quirks()
+	 * does, but it's easier to just deal with a 32-bit number since then
+	 * it can be compared with "==".
+	 *
+	 * NOTE that we deal with endianness differently for the top half
+	 * of this ID than for the bottom half. The bottom half (the product
+	 * id) gets decoded as little endian by the EDID_PRODUCT_ID because
+	 * that's how everyone seems to interpret it. The top half (the mfg_id)
+	 * gets stored as big endian because that makes
+	 * drm_edid_encode_panel_id() and drm_edid_decode_panel_id() easier
+	 * to write (it's easier to extract the ASCII). It doesn't really
+	 * matter, though, as long as the number here is unique.
+	 */
+	return (u32)edid->mfg_id[0] << 24   |
+	       (u32)edid->mfg_id[1] << 16   |
+	       (u32)EDID_PRODUCT_ID(edid);
+}
+
+/**
+ * drm_edid_get_panel_id - Get a panel's ID through DDC
+ * @adapter: I2C adapter to use for DDC
+ *
+ * This function reads the first block of the EDID of a panel and (assuming
+ * that the EDID is valid) extracts the ID out of it. The ID is a 32-bit value
+ * (16 bits of manufacturer ID and 16 bits of per-manufacturer ID) that's
+ * supposed to be different for each different modem of panel.
+ *
+ * This function is intended to be used during early probing on devices where
+ * more than one panel might be present. Because of its intended use it must
+ * assume that the EDID of the panel is correct, at least as far as the ID
+ * is concerned (in other words, we don't process any overrides here).
+ *
+ * NOTE: it's expected that this function and drm_do_get_edid() will both
+ * be read the EDID, but there is no caching between them. Since we're only
+ * reading the first block, hopefully this extra overhead won't be too big.
+ *
+ * Return: A 32-bit ID that should be different for each make/model of panel.
+ *         See the functions drm_edid_encode_panel_id() and
+ *         drm_edid_decode_panel_id() for some details on the structure of this
+ *         ID.
+ */
+
+u32 drm_edid_get_panel_id(struct i2c_adapter *adapter)
+{
+	struct edid *edid;
+	u32 panel_id;
+
+	edid = drm_do_get_edid_base_block(drm_do_probe_ddc_edid, adapter,
+					  NULL, NULL);
+
+	/*
+	 * There are no manufacturer IDs of 0, so if there is a problem reading
+	 * the EDID then we'll just return 0.
+	 */
+	if (IS_ERR_OR_NULL(edid))
+		return 0;
+
+	panel_id = edid_extract_panel_id(edid);
+	kfree(edid);
+
+	return panel_id;
+}
+EXPORT_SYMBOL(drm_edid_get_panel_id);
+
 /**
  * drm_get_edid_switcheroo - get EDID data for a vga_switcheroo output
  * @connector: connector we're probing