On Wed, 28 Jan 2015, Daniel Vetter <daniel@xxxxxxxx> wrote:
> On Wed, Jan 28, 2015 at 10:59:06AM +0200, Jani Nikula wrote:
>> On Tue, 27 Jan 2015, Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> 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.
No such thing, maybe you mean module_param_unsafe?
Jani.
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
--
Jani Nikula, Intel Open Source Technology Center
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