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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 27 Jan 2015, Ville Syrjälä <ville.syrjala@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.


BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center
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