[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 #13 from Ville Syrjala <ville.syrjala at linux.intel.com> ---
(In reply to Simon Farnsworth from comment #12)
> 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.

I don't think we should need to reduce the transfer size. What we should do is
figure out how long the transfer might reasonably take, and adjust our timeouts
accordingly.

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