Re: [PATCH 3/3] drm/i915: add i915.dp_link_train_policy option

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

On Fri, 25 Apr 2014, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Thu, Apr 24, 2014 at 06:22:59PM -0300, Paulo Zanoni wrote:
>> From: Paulo Zanoni <paulo.r.zanoni@intel.com>
>> 
>> We still have way too many bugs with DP link training. We keep
>> switching between "narrow and fast" and "wide and slow", we recently
>> added 5GHz support, and whenever there's a bug report, we have to ask
>> people to apply patches and test them.
>> 
>> Wouldn't it be so much better if we could just ask them to boot with
>> some specific Kernel boot option instead of applying a patch? This
>> will move the situation from "i915.ko is completely broken!" to
>> "i915.ko's default values are broken, but there's an option I can set
>> to fix it, so I won't need to learn how to compile a Kernel!".
>> 
>> Some useful values:
>>  - i915.dp_link_train_policy=1 for "wide and slow"
>>  - i915.dp_link_train_policy=0x120 for DP_LINK_BW_2_7 and 2 lanes,
>>    which should be able to fit 1920x1080@60Hz and 24bpp
>>  - i915.dp_link_train_policy=0x210 to force DP 5GHz testing on
>>    not-so-huge modes
>> 
>> The default behavior is still the same.
>> 
>> Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
>
> Yeah, I like this. I'll sign up Todd to review this all.

I guess we'll go with this then, but I'll step back from this particular
patch for a bit, and share my concerns over module parameters and
quirks.

I am generally opposed to adding module parameters or quirks to
workaround issues in features that should just work. They are an easy
way out for things we should root cause and fix properly.

Do not mistake me for an idealist for thinking this way, as I'm being
pragmatic.

The people who report bugs to us are roughly the same people who are
capable of setting the module parameter. Once they figure that out,
they'll stop responding to our requests for testing and info. We've seen
this happen before. We'd hurt our chances of making things work out of
the box for the average user.

The more we add module parameters, the combinations of them
explode. Debugging *other* problems becomes harder. In the bugs I work
on, the #1 request I have is full dmesg, partially because I want to see
all the wild kernel parameters the user might have set. And all too
often they have. When there are module parameters that fix some bugs,
the blogs and forums get filled with tips about them, and people use
them, whether they strictly have the same bug or not. Search for i915
invert brightness for example.

It's also not easy to drop module parameters after we've added them. You
know the drill. Even after we've fixed everything the module parameter
was supposed to fix, dropping it leads to https://xkcd.com/1172/.


BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center