Re: [Intel-gfx] [PATCH v6 2/7] drm/i915: update the QGV point frequency calculations

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

On Tue, 23 May 2023, "Govindapillai, Vinod" <vinod.govindapillai@intel.com> wrote:
> On Tue, 2023-05-23 at 12:01 +0300, Jani Nikula wrote:
>> On Tue, 23 May 2023, Vinod Govindapillai <vinod.govindapillai@intel.com> wrote:
>> > From MTL onwwards, pcode locks the QGV point based on peak BW of
>> > the intended QGV point passed by the driver. So the peak BW
>> > calculation must match the value expected by the pcode. Update
>> > the calculations as per the Bspec.
>> > 
>> > v2: use DIV_ROUND_* macro for the calculations (Ville)
>> > 
>> > Bspec: 64636
>> > 
>> > Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
>> > Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
>> > ---
>> >  drivers/gpu/drm/i915/display/intel_bw.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> > 
>> > diff --git a/drivers/gpu/drm/i915/display/intel_bw.c b/drivers/gpu/drm/i915/display/intel_bw.c
>> > index ab405c48ca3a..c8075a37c3ab 100644
>> > --- a/drivers/gpu/drm/i915/display/intel_bw.c
>> > +++ b/drivers/gpu/drm/i915/display/intel_bw.c
>> > @@ -182,7 +182,7 @@ static int mtl_read_qgv_point_info(struct drm_i915_private *dev_priv,
>> >         val2 = intel_uncore_read(&dev_priv->uncore,
>> >                                  MTL_MEM_SS_INFO_QGV_POINT_HIGH(point));
>> >         dclk = REG_FIELD_GET(MTL_DCLK_MASK, val);
>> > -       sp->dclk = DIV_ROUND_UP((16667 * dclk), 1000);
>> > +       sp->dclk = DIV_ROUND_CLOSEST(16667 * dclk + 500, 1000);
>> 
>> What's with the +500 there?
>
> This is what pcode expects. Somehow pcode use this formula and we have to exactly match this. Got
> this confirmed from Art.

I'm guessing all it really means is to round to closest, and they
specified it like that instead of saying "round to closest".

Essentially (x + (div/2)) / div is what DIV_ROUND_CLOSEST() does.

It's odd to do it *twice*, and surely not what they expect.

BR,
Jani.

>
> BR
> Vinod
>
>> 
>> BR,
>> Jani.
>> 
>> 
>> >         sp->t_rp = REG_FIELD_GET(MTL_TRP_MASK, val);
>> >         sp->t_rcd = REG_FIELD_GET(MTL_TRCD_MASK, val);
>> 
>

-- 
Jani Nikula, Intel Open Source Graphics Center