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authorSimon Farnsworth <simon.farnsworth@onelan.co.uk>2015-02-10 18:38:08 +0000
committerDaniel Vetter <daniel.vetter@ffwll.ch>2015-03-11 22:04:56 +0100
commit1d002fa720738bcd0bddb9178e9ea0773288e1dd (patch)
tree4a01b874d3b25afd098209c5983e23f50b6b3bc2 /include/drm
parent967667fd6d96a16fd4a216251eee112a214207ec (diff)
downloadlinux-1d002fa720738bcd0bddb9178e9ea0773288e1dd.tar.bz2
drm/dp: Use large transactions for I2C over AUX
Older DisplayPort to DVI-D Dual Link adapters designed by Bizlink have bugs in their I2C over AUX implementation (fixed in newer revisions). 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. Also provide an unsafe module parameter for testing smaller transfer sizes, in case there are sinks out there that cannot work with Windows. Note also that despite the previous comment in drm_dp_i2c_xfer, this speeds up native DP EDID reads; Ville Syrjälä <ville.syrjala@linux.intel.com> found the following changes in his testing: Device under test: old -> with this patch DP->DVI (OUI 001cf8): 40ms -> 35ms DP->VGA (OUI 0022b9): 45ms -> 38ms Zotac DP->2xHDMI: 25ms -> 4ms Asus PB278 monitor: 22ms -> 3ms A back of the envelope calculation shows that peak theoretical transfer rate for 1 byte reads is around 60 kbit/s; with 16 byte reads, this increases to around 500 kbit/s, which explains the increase in speed. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55228 Tested-by: Aidan Marks <aidanamarks@gmail.com> (v3) Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Diffstat (limited to 'include/drm')
-rw-r--r--include/drm/drm_dp_helper.h5
1 files changed, 5 insertions, 0 deletions
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index c5fdc2d3ca97..523f04c90dea 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
@@ -680,6 +682,9 @@ 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