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

[Daniel Vetter]

On Wed, Jan 28, 2015 at 10:59:06AM +0200, Jani Nikula wrote:
> On Tue, 27 Jan 2015, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
> > So I've been experimenting a bit with various dongles here, and sadly I've
> > not managed to get any one of them to return short reads :(
> >
> > I did find one that allows changing the speed of the i2c bus, but even if
> > I reduce it to 1khz there are no short reads, just a lot more defers. The
> > dongle in question has OUI 001cf8.
> >
> > However the good news is that EDID reads seem to get faster across the
> > board with 16 byte messages. How much faster depends on the dongle.
> >
> > Here are my measurements how long it took to read a single EDID block:
> >  DP->DVI (OUI 001cf8):	40ms -> 35ms
> >  DP->VGA (OUI 0022b9):	45ms -> 38ms
> >  Zotac DP->2xHDMI:	25ms ->  4ms
> >
> >
> > Oh and this is how I mangled my drm_dp_i2c_xfer():
> > transferred = 0;
> > while (msgs[i].len > transferred) {
> > 	msg.buffer = msgs[i].buf + transferred;
> > 	msg.size = min_t(unsigned int, drm_dp_i2c_msg_size,
> > 			 msgs[i].len - transferred);
> > 	err = drm_dp_i2c_do_msg(aux, &msg);
> > 	if (err < 0)
> > 		break;
> > 	WARN_ON(err == 0);
> > 	transferred += err;
> > }
> >
> > I made the msg size configurable via a module param just to help me test
> > this stuff, but I'm thinking we might want to upstream that just to make
> > it easier to try smaller message sizes if/when people encounter problematic
> > sinks/dongles.
> 
> How about just letting that happen first, to see if and how the problems
> occur? If there's a pattern, maybe we can fall back to 1-byte transfers
> in those cases (or even add OUI based quirks). I've grown really
> hesitant about adding new module parameters, they are ABI we can't
> easily remove/regress once added.

module_param_debug takes care of any such risks imo.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch