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

[Daniel Vetter]

On Wed, Jan 28, 2015 at 11:09:21AM +0200, Jani Nikula wrote:
> On Tue, 27 Jan 2015, Simon Farnsworth <simon.farnsworth at onelan.co.uk> 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.
> 
> IMO module parameters are ABI we shouldn't add just because we might
> need them. Also see my reply in the other thread
> http://mid.gmane.org/877fw7s2lh.fsf@intel.com

Imo just marking it as _unsafe is good enough to make it clear that it's
for debugging only.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch