[Bug 81258] Hardware polling takes over a second with DisplayPort connected

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=81258

--- Comment #8 from Todd Previte <tprevite at gmail.com> ---
Can you switch your monitor back to 1.1a and see if it still issues all those
defers?

>From the log, it looks like we're spending an inordinate amount of time waiting
for the display to respond to the EDID request(s). From the beginning to the
end of the first block of defers if about 0.65 seconds. That's just for the
first EDID read. 

Part of the reason for this is the number of retries and the transaction delays
required by the Displayport specification. Each deferred transaction must be
reattempted 7 times and each time it waits the normal 400us plus an additional
500-600us to give the sink more time to respond. So each defer costs roughly
1ms of time. A minimum of 8 transactions is required to read an entire 128-byte
EDID block (16 bytes maximum per I2C-over-AUX transaction). However, the sink
device is not required to reply with all of the data for the transaction. It
can ACK with partial data and continue the transaction on the next request. So
what should have been a single 16-byte transaction could turn into five 3-byte
transactions and one 1-byte transaction. Each of these transactions can also be
deferred up to 7 times, so those individual transaction delays that seem
somewhat insignificant can really start to add up.

-T

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