Implements an I2C-over-AUX I2C adapter on top of the generic drm_dp_aux
infrastructure. It extracts the retry logic from existing drivers, which
should help in porting those drivers to this new helper.
Signed-off-by: Thierry Reding <tred...@nvidia.com>
---
Changes in v3:
- add back DRM_DEBUG_KMS and DRM_ERROR messages
- embed i2c_adapter within struct drm_dp_aux
- fix typo in comment
drivers/gpu/drm/drm_dp_helper.c | 183 ++++++++++++++++++++++++++++++++++++++++
include/drm/drm_dp_helper.h | 5 ++
2 files changed, 188 insertions(+)
diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index ac2e12789634..dc25f63bbb0b 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -364,6 +364,13 @@ EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);
*
* The .dev field should be set to a pointer to the device that implements
* the AUX channel.
+ *
+ * An AUX channel can also be used to transport I2C messages to a sink. A
+ * typical application of that is to access an EDID that's present in the
+ * sink device. The .transfer() function can also be used to execute such
+ * transactions. The drm_dp_aux_register_i2c_bus() function registers an
+ * I2C adapter that can be passed to drm_probe_ddc(). Upon removal, drivers
+ * should call drm_dp_aux_unregister_i2c_bus() to remove the I2C adapter.
*/
static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
@@ -566,3 +573,179 @@ int drm_dp_link_configure(struct drm_dp_aux *aux, struct
drm_dp_link *link)
return 0;
}
+
+/*
+ * I2C-over-AUX implementation
+ */
+
+static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter)
+{
+ return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL |
+ I2C_FUNC_SMBUS_READ_BLOCK_DATA |
+ I2C_FUNC_SMBUS_BLOCK_PROC_CALL |
+ I2C_FUNC_10BIT_ADDR;
+}
+
+/*
+ * Transfer a single I2C-over-AUX message and handle various error conditions,
+ * retrying the transaction as appropriate.
+ */
+static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg
*msg)
+{
+ unsigned int retry;
+ int err;
+
+ /*
+ * DP1.2 sections 2.7.7.1.5.6.1 and 2.7.7.1.6.6.1: A DP Source device
+ * is required to retry at least seven times upon receiving AUX_DEFER
+ * before giving up the AUX transaction.
+ */
+ for (retry = 0; retry < 7; retry++) {
+ err = aux->transfer(aux, msg);
+ if (err < 0) {
+ if (err == -EBUSY)
+ continue;
+
+ DRM_DEBUG_KMS("transaction failed: %d\n", err);
+ return err;
+ }
+
+ switch (msg->reply & DP_AUX_NATIVE_REPLY_MASK) {
+ case DP_AUX_NATIVE_REPLY_ACK:
+ /*
+ * For I2C-over-AUX transactions this isn't enough, we
+ * need to check for the I2C ACK reply.
+ */
+ break;
+
+ case DP_AUX_NATIVE_REPLY_NACK:
+ DRM_DEBUG_KMS("native nack\n");
+ return -EREMOTEIO;
+
+ case DP_AUX_NATIVE_REPLY_DEFER:
+ DRM_DEBUG_KMS("native defer");
+ /*
+ * We could check for I2C bitrate capabilities and if
+ * available adjust this interval. We could also be
+ * more careful with DP-to-legacy adapters where a
+ * long legacy cable may force very low I2C bit rates.
+ *
+ * For now just defer for long enough to hopefully be
+ * safe for all use-cases.
+ */
+ usleep_range(500, 600);
+ continue;
+
+ default:
+ DRM_ERROR("invalid native reply %#04x\n", msg->reply);
+ return -EREMOTEIO;
+ }
+
+ switch (msg->reply & DP_AUX_I2C_REPLY_MASK) {
+ case DP_AUX_I2C_REPLY_ACK:
+ /*
+ * Both native ACK and I2C ACK replies received. We
+ * can assume the transfer was successful.
+ */
+ return 0;
+
+ case DP_AUX_I2C_REPLY_NACK:
+ DRM_DEBUG_KMS("I2C nack\n");
+ return -EREMOTEIO;
+
+ case DP_AUX_I2C_REPLY_DEFER:
+ DRM_DEBUG_KMS("I2C defer\n");
+ usleep_range(400, 500);
+ continue;
+
+ default:
+ DRM_ERROR("invalid I2C reply %#04x\n", msg->reply);
+ return -EREMOTEIO;
+ }
+ }
+
+ DRM_ERROR("too many retries, giving up\n");
+ return -EREMOTEIO;
+}
+
+static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
+ int num)
+{
+ struct drm_dp_aux *aux = adapter->algo_data;
+ unsigned int i, j;
+
+ for (i = 0; i < num; i++) {
+ struct drm_dp_aux_msg msg;
+ int err;
+
+ /*
+ * Many hardware implementations support FIFOs larger than a
+ * single byte, but it has been empirically determined that
+ * transferring data in larger chunks can actually lead to
+ * decreased performance. Therefore each message is simply
+ * transferred byte-by-byte.
+ */
+ for (j = 0; j < msgs[i].len; j++) {
+ memset(&msg, 0, sizeof(msg));
+ msg.address = msgs[i].addr;
+
+ msg.request = (msgs[i].flags & I2C_M_RD) ?
+ DP_AUX_I2C_READ :
+ DP_AUX_I2C_WRITE;
+
+ /*
+ * All messages except the last one are middle-of-
+ * transfer messages.
+ */
+ if (j < msgs[i].len - 1)
+ msg.request |= DP_AUX_I2C_MOT;
+
+ msg.buffer = msgs[i].buf + j;
+ msg.size = 1;
+
+ err = drm_dp_i2c_do_msg(aux, &msg);
+ if (err < 0)
+ return err;
+ }
+ }
+
+ return num;
+}
+
+static const struct i2c_algorithm drm_dp_i2c_algo = {
+ .functionality = drm_dp_i2c_functionality,
+ .master_xfer = drm_dp_i2c_xfer,
+};
+
+/**
+ * drm_dp_aux_register_i2c_bus() - register an I2C adapter for I2C-over-AUX
+ * @aux: DisplayPort AUX channel
+ *
+ * Returns 0 on success or a negative error code on failure.
+ */
+int drm_dp_aux_register_i2c_bus(struct drm_dp_aux *aux)
+{
+ aux->ddc.algo = &drm_dp_i2c_algo;
+ aux->ddc.algo_data = aux;
+ aux->ddc.retries = 3;
+
+ aux->ddc.class = I2C_CLASS_DDC;
+ aux->ddc.owner = THIS_MODULE;
+ aux->ddc.dev.parent = aux->dev;
+ aux->ddc.dev.of_node = aux->dev->of_node;
+
+ strncpy(aux->ddc.name, dev_name(aux->dev), sizeof(aux->ddc.name));
+
+ return i2c_add_adapter(&aux->ddc);
+}
+EXPORT_SYMBOL(drm_dp_aux_register_i2c_bus);
+
+/**
+ * drm_dp_aux_unregister_i2c_bus() - unregister an I2C-over-AUX adapter
+ * @aux: DisplayPort AUX channel
+ */
+void drm_dp_aux_unregister_i2c_bus(struct drm_dp_aux *aux)
+{
+ i2c_del_adapter(&aux->ddc);
+}
+EXPORT_SYMBOL(drm_dp_aux_unregister_i2c_bus);
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index c7b3c736c40a..4d67bd12089b 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -421,10 +421,12 @@ struct drm_dp_aux_msg {
/**
* struct drm_dp_aux - DisplayPort AUX channel
+ * @ddc: I2C adapter that can be used for I2C-over-AUX communication
* @dev: pointer to struct device that is the parent for this AUX channel
* @transfer: transfers a message representing a single AUX transaction
*/
struct drm_dp_aux {
+ struct i2c_adapter ddc;
struct device *dev;
ssize_t (*transfer)(struct drm_dp_aux *aux,
@@ -485,4 +487,7 @@ int drm_dp_link_probe(struct drm_dp_aux *aux, struct
drm_dp_link *link);
int drm_dp_link_power_up(struct drm_dp_aux *aux, struct drm_dp_link *link);
int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link);
+int drm_dp_aux_register_i2c_bus(struct drm_dp_aux *aux);
+void drm_dp_aux_unregister_i2c_bus(struct drm_dp_aux *aux);
+
#endif /* _DRM_DP_HELPER_H_ */
--
1.8.4.2
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