Re: [PATCH 1/6] drm/i915/crt: don't set HOTPLUG bits on !PCH

From: Paulo Zanoni <przanoni@gmail.com>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Intel Graphics Development <intel-gfx@lists.freedesktop.org>
Subject: Re: [PATCH 1/6] drm/i915/crt: don't set HOTPLUG bits on !PCH
Date: Fri, 12 Oct 2012 14:17:11 -0300	[thread overview]
Message-ID: <CA+gsUGQm3S=eTS6f7+M8F+KG7fhuq6AimD2ZwHJ-3LcEqqt98w@mail.gmail.com> (raw)
In-Reply-To: <CAKMK7uGq9oXsZQQFZxardAwpOxHsu+D=DvEUrD70HLcinvQwBQ@mail.gmail.com>

2012/10/12 Daniel Vetter <daniel.vetter@ffwll.ch>:
> On Fri, Oct 12, 2012 at 4:47 AM, Paulo Zanoni <przanoni@gmail.com> wrote:
>> 2012/10/11 Daniel Vetter <daniel.vetter@ffwll.ch>:
>>> ... since they don't apply to pre-pch platforms and could actually be
>>> harmful.
>>>
>>> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>>
>> Ok, so I checked the specs and yes, these bit definitions don't exist.
>> The problem here is that instead of "must-be-zero", the spec says
>> "Reserved. Software must preserve the contents of these bits" for bits
>> 29:16 (and also some others). So maybe by setting everything to 0
>> instead of enabling bits 17, 18, 20-23 we could actually be breaking
>> things? Either way, both the old and new code don't follow the
>> specification.
>
> Indeed, I've overlooked the "must be preserved" wording, and it goes
> back to gen2 when the ADPA reg was added. So I've fired up all my
> gen2/gen3 machines, and they have all zeros in these registers.

Ok, so please do a final test: try to write something to those
"must-be-preserved" bits and check if the values stay or not. If after
writing 1 to bits 17-18, 20-23 you read 0, then you have my
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>. I still do plan
to test the 6 patches of the series on hsw later btw.

> And we
> never write anything in there. I suspect this is simply a hint from
> the Bspec authors to driver writes that they might eventually use
> these reserved bits in future hw platforms. And if the driver
> preserves the bit settings, it will automatically work. I think MBZ is
> mostly used for bit ranges that have been used once, but are no longer
> implemented (e.g. bits 12:13 do something in gen2/3 but not on later
> hw gens).
>
>> Maybe on non-pch-split we could try to read ADPA and erase all the
>> bits except the "must-be-preserved" ones?
>
> See above, I think we can clear them - at least all my old hw seems to
> work perfectly well with all these bits being zero.
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

-- 
Paulo Zanoni