drm/display: Don't assume dual mode adaptors support i2c sub-addressing

Current dual mode adaptor ("DP++") detection code assumes that all
adaptors support i2c sub-addressing for read operations from the
DP-HDMI adaptor ID buffer. It has been observed that multiple
adaptors do not in fact support this, and always return data starting
at register 0. On affected adaptors, the code fails to read the proper
registers that would identify the device as a type 2 adaptor, and
handles those as type 1, limiting the TMDS clock to 165MHz, even if
the according register would announce a higher TMDS clock.
Fix this by always reading the ID buffer starting from offset 0, and
discarding any bytes before the actual offset of interest.

We tried finding authoritative documentation on whether or not this is
allowed behaviour, but since all the official VESA docs are paywalled,
the best we could come up with was the spec sheet for Texas Instruments'
SNx5DP149 chip family.[1] It explicitly mentions that sub-addressing is
supported for register writes, but *not* for reads (See NOTE in
section 8.5.3). Unless TI openly decided to violate the VESA spec, one
could take that as a hint that sub-addressing is in fact not mandated
by VESA.
The other two adaptors affected used the PS8409(A) and the LT8611,
according to the data returned from their ID buffers.

[1] https://www.ti.com/lit/ds/symlink/sn75dp149.pdf

Cc: [e-mail]
Signed-off-by: Simon Rettberg <[e-mail]>
Reviewed-by: Rafael Gieschke <[e-mail]>
Signed-off-by: Ville Syrjälä <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006113314.41101987@computer
Acked-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/display/drm_dp_dual_mode_helper.c

···
 ssize_t drm_dp_dual_mode_read(struct i2c_adapter *adapter,
 			      u8 offset, void *buffer, size_t size)
 {
+	u8 zero = 0;
+	char *tmpbuf = NULL;
+	/*
+	 * As sub-addressing is not supported by all adaptors,
+	 * always explicitly read from the start and discard
+	 * any bytes that come before the requested offset.
+	 * This way, no matter whether the adaptor supports it
+	 * or not, we'll end up reading the proper data.
+	 */
 	struct i2c_msg msgs[] = {
 		{
 			.addr = DP_DUAL_MODE_SLAVE_ADDRESS,
 			.flags = 0,
 			.len = 1,
-			.buf = &offset,
+			.buf = &zero,
 		},
 		{
 			.addr = DP_DUAL_MODE_SLAVE_ADDRESS,
 			.flags = I2C_M_RD,
-			.len = size,
+			.len = size + offset,
 			.buf = buffer,
 		},
 	};
 	int ret;
 
+	if (offset) {
+		tmpbuf = kmalloc(size + offset, GFP_KERNEL);
+		if (!tmpbuf)
+			return -ENOMEM;
+
+		msgs[1].buf = tmpbuf;
+	}
+
 	ret = i2c_transfer(adapter, msgs, ARRAY_SIZE(msgs));
+	if (tmpbuf)
+		memcpy(buffer, tmpbuf + offset, size);
+
+	kfree(tmpbuf);
+
 	if (ret < 0)
 		return ret;
 	if (ret != ARRAY_SIZE(msgs))
···
 	if (ret)
 		return DRM_DP_DUAL_MODE_UNKNOWN;
 
-	/*
-	 * Sigh. Some (maybe all?) type 1 adaptors are broken and ack
-	 * the offset but ignore it, and instead they just always return
-	 * data from the start of the HDMI ID buffer. So for a broken
-	 * type 1 HDMI adaptor a single byte read will always give us
-	 * 0x44, and for a type 1 DVI adaptor it should give 0x00
-	 * (assuming it implements any registers). Fortunately neither
-	 * of those values will match the type 2 signature of the
-	 * DP_DUAL_MODE_ADAPTOR_ID register so we can proceed with
-	 * the type 2 adaptor detection safely even in the presence
-	 * of broken type 1 adaptors.
-	 */
 	ret = drm_dp_dual_mode_read(adapter, DP_DUAL_MODE_ADAPTOR_ID,
 				    &adaptor_id, sizeof(adaptor_id));
 	drm_dbg_kms(dev, "DP dual mode adaptor ID: %02x (err %zd)\n", adaptor_id, ret);
···
 				return DRM_DP_DUAL_MODE_TYPE2_DVI;
 		}
 		/*
-		 * If neither a proper type 1 ID nor a broken type 1 adaptor
-		 * as described above, assume type 1, but let the user know
-		 * that we may have misdetected the type.
+		 * If not a proper type 1 ID, still assume type 1, but let
+		 * the user know that we may have misdetected the type.
 		 */
-		if (!is_type1_adaptor(adaptor_id) && adaptor_id != hdmi_id[0])
+		if (!is_type1_adaptor(adaptor_id))
 			drm_err(dev, "Unexpected DP dual mode adaptor ID %02x\n", adaptor_id);
 
 	}
···
  * @enable: enable (as opposed to disable) the TMDS output buffers
  *
  * Set the state of the TMDS output buffers in the adaptor. For
- * type2 this is set via the DP_DUAL_MODE_TMDS_OEN register. As
- * some type 1 adaptors have problems with registers (see comments
- * in drm_dp_dual_mode_detect()) we avoid touching the register,
- * making this function a no-op on type 1 adaptors.
+ * type2 this is set via the DP_DUAL_MODE_TMDS_OEN register.
+ * Type1 adaptors do not support any register writes.
  *
  * Returns:
  * 0 on success, negative error code on failure