Re: [Intel-gfx] [PATCH 1/9] drm/i915: Bump DSL linemask to 20 bits

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

On Fri, 19 Nov 2021, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> On Mon, Nov 15, 2021 at 02:05:00PM -0500, Rodrigo Vivi wrote:
>> On Fri, Nov 12, 2021 at 09:38:05PM +0200, Ville Syrjala wrote:
>> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> > 
>> > Since tgl PIPE_DSL has 20 bits for the scanline. Let's bump our
>> > definition to match. And while at it let's also add the define
>> > for the current field readback.
>> > 
>> > We can also get rid of the gen2 vs. gen3+ nonsense since none
>> > of the extra bits ever did anything and just always read
>> > as zero.
>> 
>> You are stepping over reserved bits on older platforms here.
>> 
>> I understand that must probably hw is not using this for anything
>> and the reads are only zero. But I'm always afraid of opening
>> precedence for this kind of assumptions and end up stepping
>> over some reserved bit that hw is using for something else
>> but not documented.
>
> We do this in other places too in order to keep the code
> simple. I think it's fine for cases where all old platforms
> had a smaller bitfield which is extended in later platforms.
> That is, assuming all the bits were unused (and read as zero)
> in the old platforms, which is the case here.
>
> The thing we probably shouldn't do is make the bitfield larger
> proactively for future platforms since we can't know if some of
> the currently unused bits might end up being used for something
> else in the future.
>
> I really hope we don't have any undocumented bits anywhere since
> those can screw us up in a lot more ways than this. If we do find
> any undocuemnted bits we really need to file bspec issues for those.

I guess I'd record some of this in the commit message while applying, in
case this blows up. Other than that,

Reviewed-by: Jani Nikula <jani.nikula@intel.com>


-- 
Jani Nikula, Intel Open Source Graphics Center