[Bug 91451] Apple miniDP-to-VGA adapter doesn't work anymore since Kernel 4.1

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91451

--- Comment #12 from Simon Farnsworth <simon at farnz.org.uk> ---
So, a suggestion if anyone's got time to work on this:

drm_kms_helper.dp_aux_i2c_transfer_size sets the number of bytes of I2C data we
try to transfer in a single DP AUX transaction; the maximum is 16, the minimum
is 1 byte. I have devices that need 16 bytes per transaction, otherwise they
fail; it looks like the Apple adapter fails if you try to transfer more than 4
bytes per transaction.

drm_dp_i2c_xfer currently only reduces the transfer size if the DP device
returns a partial success (e.g. 4 bytes of I2C data returned when we asked for
16 will cause us to try with 4 bytes per transaction in future). With a bit of
care, it could also learn that when the I2C transaction reaches the retry limit
and the only response to DP AUX transactions was DP_AUX_NATIVE_REPLY_DEFER,
it's time to reduce the transaction size. Note that you do expect some
DP_AUX_NATIVE_REPLY_DEFERs in normal operation - they're how the device tells
you that it can't reply yet.

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