[PATCH v3 1/2] drm/dp: Factor out a function to probe a DPCD address

[Imre Deak <imre.deak@intel.com>]

Factor out from drm_dp_dpcd_read() a function to probe a DPCD address
with a 1-byte read access. This will be needed by the next patch doing a
read from an LTTPR address, which must happen without the preceding
wake-up read in drm_dp_dpcd_read().

While at it add tracing for the 1 byte read even if the read was
successful.

v2: Add a probe function instead of exporting drm_dp_dpcd_access(). (Jani)
v3: Add tracing for the 1-byte read even if the read was successful. (Khaled)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Khaled Almahallawy <khaled.almahallawy@intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/dp/drm_dp.c    | 33 ++++++++++++++++++++++++++++-----
 include/drm/dp/drm_dp_helper.h |  1 +
 2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/dp/drm_dp.c b/drivers/gpu/drm/dp/drm_dp.c
index 580016a1b9eb7..2a1f5ff6633cc 100644
--- a/drivers/gpu/drm/dp/drm_dp.c
+++ b/drivers/gpu/drm/dp/drm_dp.c
@@ -527,6 +527,31 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 	return ret;
 }
 
+/**
+ * drm_dp_dpcd_probe() - probe a given DPCD address with a 1-byte read access
+ * @aux: DisplayPort AUX channel (SST)
+ * @offset: address of the register to probe
+ *
+ * Probe the provided DPCD address by reading 1 byte from it. The function can
+ * be used to trigger some side-effect the read access has, like waking up the
+ * sink, without the need for the read-out value.
+ *
+ * Returns 0 if the read access suceeded, or a negative error code on failure.
+ */
+int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset)
+{
+	u8 buffer;
+	int ret;
+
+	ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, &buffer, 1);
+	WARN_ON(ret == 0);
+
+	drm_dp_dump_access(aux, DP_AUX_NATIVE_READ, offset, &buffer, ret);
+
+	return ret < 0 ? ret : 0;
+}
+EXPORT_SYMBOL(drm_dp_dpcd_probe);
+
 /**
  * drm_dp_dpcd_read() - read a series of bytes from the DPCD
  * @aux: DisplayPort AUX channel (SST or MST)
@@ -559,10 +584,9 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 	 * monitor doesn't power down exactly after the throw away read.
 	 */
 	if (!aux->is_remote) {
-		ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, DP_DPCD_REV,
-					 buffer, 1);
-		if (ret != 1)
-			goto out;
+		ret = drm_dp_dpcd_probe(aux, DP_DPCD_REV);
+		if (ret < 0)
+			return ret;
 	}
 
 	if (aux->is_remote)
@@ -571,7 +595,6 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 		ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset,
 					 buffer, size);
 
-out:
 	drm_dp_dump_access(aux, DP_AUX_NATIVE_READ, offset, buffer, ret);
 	return ret;
 }
diff --git a/include/drm/dp/drm_dp_helper.h b/include/drm/dp/drm_dp_helper.h
index 1eccd97419436..91af98e6617c6 100644
--- a/include/drm/dp/drm_dp_helper.h
+++ b/include/drm/dp/drm_dp_helper.h
@@ -2053,6 +2053,7 @@ struct drm_dp_aux {
 	bool is_remote;
 };
 
+int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset);
 ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 			 void *buffer, size_t size);
 ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
-- 
2.30.2