On Tue, Jan 27, 2015 at 03:43:49PM +0000, Simon Farnsworth wrote: > 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 Another datapoint: Asus PB278 monitor: 22ms -> 3ms I've been pondering about these massive improvements for the Zotac and Asus. After reading the DP spec a bit more I've noticed just how wasteful AUX is. If my understanding of the spec and arithmetic aren't totally off, the effective bandwidth for EDID reads is ~400kbps when using 16 byte AUX messages, but it drops to only ~60kbps when doing 1 byte AUX messages. Those seem to match reasonably well with my measurements. I never realized just how slow it can be since the 1Mbps number feels like plenty, and no one ever seems to mention the effective bandwidth. -- Ville Syrjälä Intel OTC _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
