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2023-12-09 b22da3d8526a935aa31e086e63f60ff3246cb61c

 kernel/drivers/gpu/drm/drm_dp_dual_mode_helper.c
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@@ -20,13 +20,15 @@
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  * OTHER DEALINGS IN THE SOFTWARE.
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  */
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+#include <linux/delay.h>
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 #include <linux/errno.h>
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 #include <linux/export.h>
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 #include <linux/i2c.h>
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 #include <linux/slab.h>
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 #include <linux/string.h>
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+
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 #include <drm/drm_dp_dual_mode_helper.h>
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-#include <drm/drmP.h>
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+#include <drm/drm_print.h>
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 /**
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  * DOC: dp dual mode helpers
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@@ -60,23 +62,45 @@
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 ssize_t drm_dp_dual_mode_read(struct i2c_adapter *adapter,
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 			      u8 offset, void *buffer, size_t size)
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 {
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+	u8 zero = 0;
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+	char *tmpbuf = NULL;
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+	/*
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+	 * As sub-addressing is not supported by all adaptors,
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+	 * always explicitly read from the start and discard
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+	 * any bytes that come before the requested offset.
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+	 * This way, no matter whether the adaptor supports it
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+	 * or not, we'll end up reading the proper data.
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+	 */
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 	struct i2c_msg msgs[] = {
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 		{
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 			.addr = DP_DUAL_MODE_SLAVE_ADDRESS,
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 			.flags = 0,
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 			.len = 1,
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-			.buf = &offset,
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+			.buf = &zero,
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 		},
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 		{
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 			.addr = DP_DUAL_MODE_SLAVE_ADDRESS,
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 			.flags = I2C_M_RD,
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-			.len = size,
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+			.len = size + offset,
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 			.buf = buffer,
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 		},
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 	};
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 	int ret;
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+	if (offset) {
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+		tmpbuf = kmalloc(size + offset, GFP_KERNEL);
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+		if (!tmpbuf)
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+			return -ENOMEM;
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+
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+		msgs[1].buf = tmpbuf;
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+	}
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+
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 	ret = i2c_transfer(adapter, msgs, ARRAY_SIZE(msgs));
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+	if (tmpbuf)
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+		memcpy(buffer, tmpbuf + offset, size);
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+
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+	kfree(tmpbuf);
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+
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 	if (ret < 0)
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 		return ret;
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 	if (ret != ARRAY_SIZE(msgs))
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@@ -203,18 +227,6 @@
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 	if (ret)
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 		return DRM_DP_DUAL_MODE_UNKNOWN;
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-	/*
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-	 * Sigh. Some (maybe all?) type 1 adaptors are broken and ack
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-	 * the offset but ignore it, and instead they just always return
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-	 * data from the start of the HDMI ID buffer. So for a broken
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-	 * type 1 HDMI adaptor a single byte read will always give us
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-	 * 0x44, and for a type 1 DVI adaptor it should give 0x00
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-	 * (assuming it implements any registers). Fortunately neither
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-	 * of those values will match the type 2 signature of the
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-	 * DP_DUAL_MODE_ADAPTOR_ID register so we can proceed with
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-	 * the type 2 adaptor detection safely even in the presence
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-	 * of broken type 1 adaptors.
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-	 */
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 	ret = drm_dp_dual_mode_read(adapter, DP_DUAL_MODE_ADAPTOR_ID,
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 				    &adaptor_id, sizeof(adaptor_id));
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 	DRM_DEBUG_KMS("DP dual mode adaptor ID: %02x (err %zd)\n",
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@@ -229,11 +241,10 @@
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 				return DRM_DP_DUAL_MODE_TYPE2_DVI;
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 		}
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 		/*
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-		 * If neither a proper type 1 ID nor a broken type 1 adaptor
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-		 * as described above, assume type 1, but let the user know
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-		 * that we may have misdetected the type.
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+		 * If not a proper type 1 ID, still assume type 1, but let
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+		 * the user know that we may have misdetected the type.
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 		 */
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-		if (!is_type1_adaptor(adaptor_id) && adaptor_id != hdmi_id[0])
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+		if (!is_type1_adaptor(adaptor_id))
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 			DRM_ERROR("Unexpected DP dual mode adaptor ID %02x\n",
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 				  adaptor_id);
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@@ -337,10 +348,8 @@
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  * @enable: enable (as opposed to disable) the TMDS output buffers
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  *
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  * Set the state of the TMDS output buffers in the adaptor. For
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- * type2 this is set via the DP_DUAL_MODE_TMDS_OEN register. As
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- * some type 1 adaptors have problems with registers (see comments
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- * in drm_dp_dual_mode_detect()) we avoid touching the register,
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- * making this function a no-op on type 1 adaptors.
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+ * type2 this is set via the DP_DUAL_MODE_TMDS_OEN register.
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+ * Type1 adaptors do not support any register writes.
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  *
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  * Returns:
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  * 0 on success, negative error code on failure