1d002fa720
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> |
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