[PATCH v3] drm/dp: Use large transactions for I2C over AUX

[Ville Syrjälä]

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 at onelan.co.uk>
> ---
> 
> 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