drm/dp: Add AUX channel infrastructure

This is a superset of the current i2c_dp_aux bus functionality and can
be used to transfer native AUX in addition to I2C-over-AUX messages.

Helpers are provided to read and write the DPCD, either blockwise or
byte-wise. Many of the existing helpers for DisplayPort take a copy of a
portion of the DPCD and operate on that, without a way to write data
back to the DPCD (e.g. for configuration of the link).

Subsequent patches will build upon this infrastructure to provide common
functionality in a generic way.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
---
Changes in v5:
- move comments partially to struct drm_dp_aux_msg in header file
- return -EPROTO on short reads in DPCD helpers

Changes in v4:
- fix a typo in a comment

Changes in v3:
- reorder drm_dp_dpcd_writeb() arguments to be more intuitive
- return number of bytes transferred in drm_dp_dpcd_write()
- factor out drm_dp_dpcd_access()
- describe error codes

1 files changed, 110 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 9e978aae8972..da6bcfe178a6 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -346,3 +346,113 @@ int drm_dp_bw_code_to_link_rate(u8 link_bw)
         }
 }
 EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);
+
+/**
+ * DOC: dp helpers
+ *
+ * The DisplayPort AUX channel is an abstraction to allow generic, driver-
+ * independent access to AUX functionality. Drivers can take advantage of
+ * this by filling in the fields of the drm_dp_aux structure.
+ *
+ * Transactions are described using a hardware-independent drm_dp_aux_msg
+ * structure, which is passed into a driver's .transfer() implementation.
+ * Both native and I2C-over-AUX transactions are supported.
+ *
+ * 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,
+                              unsigned int offset, void *buffer, size_t size)
+{
+        struct drm_dp_aux_msg msg;
+        unsigned int retry;
+        int err;
+
+        memset(&msg, 0, sizeof(msg));
+        msg.address = offset;
+        msg.request = request;
+        msg.buffer = buffer;
+        msg.size = size;
+
+        /*
+         * The specification doesn't give any recommendation on how often to
+         * retry native transactions, so retry 7 times like for I2C-over-AUX
+         * transactions.
+         */
+        for (retry = 0; retry < 7; retry++) {
+                err = aux->transfer(aux, &msg);
+                if (err < 0) {
+                        if (err == -EBUSY)
+                                continue;
+
+                        return err;
+                }
+
+                if (err < size)
+                        return -EPROTO;
+
+                switch (msg.reply & DP_AUX_NATIVE_REPLY_MASK) {
+                case DP_AUX_NATIVE_REPLY_ACK:
+                        return err;
+
+                case DP_AUX_NATIVE_REPLY_NACK:
+                        return -EIO;
+
+                case DP_AUX_NATIVE_REPLY_DEFER:
+                        usleep_range(400, 500);
+                        break;
+                }
+        }
+
+        DRM_ERROR("too many retries, giving up\n");
+        return -EIO;
+}
+
+/**
+ * drm_dp_dpcd_read() - read a series of bytes from the DPCD
+ * @aux: DisplayPort AUX channel
+ * @offset: address of the (first) register to read
+ * @buffer: buffer to store the register values
+ * @size: number of bytes in @buffer
+ *
+ * Returns the number of bytes transferred on success, or a negative error
+ * code on failure. -EIO is returned if the request was NAKed by the sink or
+ * if the retry count was exceeded. If not all bytes were transferred, this
+ * function returns -EPROTO. Errors from the underlying AUX channel transfer
+ * function, with the exception of -EBUSY (which causes the transaction to
+ * be retried), are propagated to the caller.
+ */
+ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
+                         void *buffer, size_t size)
+{
+        return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, buffer,
+                                  size);
+}
+EXPORT_SYMBOL(drm_dp_dpcd_read);
+
+/**
+ * drm_dp_dpcd_write() - write a series of bytes to the DPCD
+ * @aux: DisplayPort AUX channel
+ * @offset: address of the (first) register to write
+ * @buffer: buffer containing the values to write
+ * @size: number of bytes in @buffer
+ *
+ * Returns the number of bytes transferred on success, or a negative error
+ * code on failure. -EIO is returned if the request was NAKed by the sink or
+ * if the retry count was exceeded. If not all bytes were transferred, this
+ * function returns -EPROTO. Errors from the underlying AUX channel transfer
+ * function, with the exception of -EBUSY (which causes the transaction to
+ * be retried), are propagated to the caller.
+ */
+ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
+                          void *buffer, size_t size)
+{
+        return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_WRITE, offset, buffer,
+                                  size);
+}
+EXPORT_SYMBOL(drm_dp_dpcd_write);