DisplayPort to DVI-D Dual Link adapters designed by Bizlink have bugs in
their I2C over AUX implementation. They work fine with Windows, but fail
with Linux.
It turns out that they cannot keep an I2C transaction open unless the
previous read was 16 bytes; shorter reads can only be followed by a zero
byte transfer ending the I2C transaction.
Copy Windows's behaviour, and read 16 bytes at a time. If we get a short
reply, assume that there's a hardware bottleneck, and shrink our read size
to match. For this purpose, use the algorithm in the DisplayPort 1.2 spec,
in the hopes that it'll be closest to what Windows does, as no sink I've
found actually gives short replies.
Also provide a module parameter for testing smaller transfer sizes, in case
there are sinks out there that cannot work with Windows.
Signed-off-by: Simon Farnsworth <simon.farnsworth@xxxxxxxxxxxx>
---
v3 changes, after feedback from Ville and more testing of Windows:
* Change the short reply algorithm to match Ville's description of the
DisplayPort 1.2 spec wording.
* Add a module parameter to set the default transfer size for
experiments. Requested over IRC by Ville.
No-one's been able to find a device that does short replies, but experiments
show that bigger reads are faster on most devices. Ville got:
DP->DVI (OUI 001cf8): 40ms -> 35ms
DP->VGA (OUI 0022b9): 45ms -> 38ms
Zotac DP->2xHDMI: 25ms -> 4ms
v2 changes, after feedback from Thierry and Ville:
* Handle short replies. I've decided (arbitrarily) that a short reply
results in us dropping back to the newly chosen size for the rest of this
I2C transaction. Thus, given an attempt to read the first 16 bytes of
EDID, and a sink that only does 4 bytes of buffering, we will see the
following AUX transfers for the EDID read (after address is set):
<set address, block etc>
Read 16 bytes from I2C over AUX.
Reply with 4 bytes
Read 4 bytes
Reply with 4 bytes
Read 4 bytes
Reply with 4 bytes
Read 4 bytes
Reply with 4 bytes
<end I2C transaction>
Note that I've not looked at MST support - I have neither the DP 1.2 spec
nor any MST branch devices, so I can't test anything I write or check it
against a spec. It looks from the code, however, as if MST has the branch
device do the split from a big request into small transactions.
drivers/gpu/drm/drm_dp_helper.c | 91 ++++++++++++++++++++++++++++++++---------
include/drm/drm_dp_helper.h | 5 +++
2 files changed, 77 insertions(+), 19 deletions(-)
diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 79968e3..3db116c 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -396,11 +396,13 @@ static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter)
* retrying the transaction as appropriate. It is assumed that the
* aux->transfer function does not modify anything in the msg other than the
* reply field.
+ *
+ * Returns bytes transferred on success, or a negative error code on failure.
*/
static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
{
unsigned int retry;
- int err;
+ int ret;
/*
* DP1.2 sections 2.7.7.1.5.6.1 and 2.7.7.1.6.6.1: A DP Source device
@@ -409,14 +411,14 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
*/
for (retry = 0; retry < 7; retry++) {
mutex_lock(&aux->hw_mutex);
- err = aux->transfer(aux, msg);
+ ret = aux->transfer(aux, msg);
mutex_unlock(&aux->hw_mutex);
- if (err < 0) {
- if (err == -EBUSY)
+ if (ret < 0) {
+ if (ret == -EBUSY)
continue;
- DRM_DEBUG_KMS("transaction failed: %d\n", err);
- return err;
+ DRM_DEBUG_KMS("transaction failed: %d\n", ret);
+ return ret;
}
@@ -457,9 +459,7 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
* Both native ACK and I2C ACK replies received. We
* can assume the transfer was successful.
*/
- if (err < msg->size)
- return -EPROTO;
- return 0;
+ return ret;
case DP_AUX_I2C_REPLY_NACK:
DRM_DEBUG_KMS("I2C nack\n");
@@ -482,14 +482,67 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
return -EREMOTEIO;
}
+/*
+ * Keep retrying drm_dp_i2c_do_msg until all data has been transferred.
+ *
+ * Returns an error code on failure, or a recommended transfer size on success.
+ */
+static int drm_dp_i2c_drain_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
+{
+ int ret;
+ struct drm_dp_aux_msg drain_msg;
+ int drain_bytes;
+
+ ret = drm_dp_i2c_do_msg(aux, msg);
+
+ if (ret == msg->size || ret <= 0)
+ return ret == 0 ? -EPROTO : ret;
+
+ /*
+ * We had a partial reply. Drain out the rest of the bytes we
+ * originally requested, then return the size of the partial
+ * reply. In theory, this should match DP 1.2's desired behaviour
+ * for I2C over AUX.
+ */
+ DRM_DEBUG_KMS("Partial I2C reply: requested %zu bytes got %d bytes\n", msg->size, ret);
+ drain_msg = *msg;
+ drain_msg.size -= ret;
+ drain_msg.buffer += ret;
+ while (drain_msg.size != 0) {
+ drain_bytes = drm_dp_i2c_do_msg(aux, &drain_msg);
+ if (drain_bytes <= 0)
+ return drain_bytes == 0 ? -EPROTO : drain_bytes;
+ drain_msg.size -= drain_bytes;
+ drain_msg.buffer += drain_bytes;
+ }
+ return ret;
+}
+
+/*
+ * Bizlink designed DP->DVI-D Dual Link adapters require the I2C over AUX
+ * packets to be as large as possible. If not, the I2C transactions never
+ * succeed. Hence the default is maximum.
+ */
+static int dp_aux_i2c_transfer_size __read_mostly = DP_AUX_MAX_PAYLOAD_BYTES;
+module_param_named(dp_aux_i2c_transfer_size, dp_aux_i2c_transfer_size, int, 0600);
+MODULE_PARM_DESC(dp_aux_i2c_transfer_size,
+ "Number of bytes to transfer in a single I2C over DP AUX CH message, (1-16, default 16)");
+
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;
+ int transfer_size;
struct drm_dp_aux_msg msg;
int err = 0;
+ if (dp_aux_i2c_transfer_size < 1 || dp_aux_i2c_transfer_size > DP_AUX_MAX_PAYLOAD_BYTES) {
+ DRM_ERROR("dp_aux_i2c_transfer_size invalid - changing from %d to %d\n",
+ dp_aux_i2c_transfer_size, DP_AUX_MAX_PAYLOAD_BYTES);
+ dp_aux_i2c_transfer_size = DP_AUX_MAX_PAYLOAD_BYTES;
+ }
+
memset(&msg, 0, sizeof(msg));
for (i = 0; i < num; i++) {
@@ -507,20 +560,20 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
err = drm_dp_i2c_do_msg(aux, &msg);
if (err < 0)
break;
- /*
- * 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.
+ /* We want each transaction to be as large as possible, but
+ * we'll go to smaller sizes if the hardware gives us a
+ * short reply.
*/
- for (j = 0; j < msgs[i].len; j++) {
+ transfer_size = dp_aux_i2c_transfer_size;
+ for (j = 0; j < msgs[i].len; j += msg.size) {
msg.buffer = msgs[i].buf + j;
- msg.size = 1;
+ msg.size = min((unsigned)transfer_size, msgs[i].len - j);
- err = drm_dp_i2c_do_msg(aux, &msg);
- if (err < 0)
+ transfer_size = drm_dp_i2c_drain_msg(aux, &msg);
+ if (transfer_size < 0) {
+ err = transfer_size;
break;
+ }
}
if (err < 0)
break;
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 11f8c84..444d51b 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -42,6 +42,8 @@
* 1.2 formally includes both eDP and DPI definitions.
*/
+#define DP_AUX_MAX_PAYLOAD_BYTES 16
+
#define DP_AUX_I2C_WRITE 0x0
#define DP_AUX_I2C_READ 0x1
#define DP_AUX_I2C_STATUS 0x2
@@ -519,6 +521,9 @@ struct drm_dp_aux_msg {
* 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.
+ * The I2C adapter uses long transfers by default; if a partial response is
+ * received, the adapter will drop down to the size given by the partial
+ * response for this transaction only.
*
* Note that the aux helper code assumes that the .transfer() function
* only modifies the reply field of the drm_dp_aux_msg structure. The
--
2.1.0
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