[Intel-gfx] [PATCH 1/2] drm/i915: Re-add enable_rc6 modparam

[Tvrtko Ursulin]

On 15/05/2019 01:06, Rodrigo Vivi wrote:
> On Tue, May 14, 2019 at 06:32:01PM +0000, Summers, Stuart wrote:
>> On Tue, 2019-05-14 at 17:53 +0100, Chris Wilson wrote:
>>> Quoting Stuart Summers (2019-05-14 17:46:52)
>>>> To allow easier debug of platforms which do not fully support
>>>> power-saving render C-state 6, add back the module parameter
>>>> to allow RC6 flows to be disabled. Instead of directly affecting
>>>> the RC6 states via a bitmask as done previously, use this module
>>>> parameter to clear the has_rc6 field for these platforms.
>>>
>>> If you know which platforms don't support rc6, don't enable rc6.
>>
>> I'd really prefer to have this parameter in place for debug purposes.
>> It seems more useful to allow quick testing by reloading i915 than by
>> requiring a rebuild.
>>
>> Of course, once debug is complete and the platform is known to either
>> not support the feature or has some cripling bug around this, I agree,
>> rc6 shouldn't be enabled on that platform and i915 should be updated.
> 
> Exactly. We need the flexibility for debug that.
> unfortunately using debugfs doesn't look a solution.
> 
> One possibility that just came to my mind now is, what if we make
> this only for platforms that are still protected by is_alpha_support=1
> (soon becoming require_force_probe=1)
> 
> But this is just one side of the coin... when product is out there
> and we want the user to debug the issue to see if it is a RC6 bug
> we have no way to verify that. :/
> 
> Please let's add this back one way or another.

To throw a compromise option out there - add the modparam for debug 
builds only (CONFIG_DRM_I915_DEBUG)?

Regards,

Tvrtko