[Intel-gfx] [PATCH 3/3] drm/i915: Use the ceil value for the additional clk divider

[Deepak, M]

> -----Original Message-----
> From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com]
> Sent: Friday, December 4, 2015 5:22 PM
> To: Deepak, M <m.deepak at intel.com>
> Cc: intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH 3/3] drm/i915: Use the ceil value for the
> additional clk divider
> 
> On Fri, Dec 04, 2015 at 07:47:39PM +0530, Deepak M wrote:
> > Additional clock value divider should use the ceil value of the
> > calulation to get the correct divider value.
> >
> > Signed-off-by: Deepak M <m.deepak at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dsi_pll.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_dsi_pll.c
> > b/drivers/gpu/drm/i915/intel_dsi_pll.c
> > index cb3cf39..1322a71 100644
> > --- a/drivers/gpu/drm/i915/intel_dsi_pll.c
> > +++ b/drivers/gpu/drm/i915/intel_dsi_pll.c
> > @@ -454,7 +454,7 @@ static void bxt_dsi_program_clocks(struct
> drm_device *dev, enum port port)
> >  	dsi_rate = (BXT_REF_CLOCK_KHZ * pll_ratio) / 2;
> >
> >  	/* Max possible output of clock is 39.5 MHz, program value -1 */
> > -	divider = (dsi_rate / BXT_MAX_VAR_OUTPUT_KHZ) - 1;
> > +	divider = DIV_ROUND_UP(dsi_rate, BXT_MAX_VAR_OUTPUT_KHZ) -
> 1;
> 
> I can't find anything to support the 39.5 MHz claim above. I do know the tx
> escape clock should be <=20Mhz, so with the /2 extra divider it seems we
> should aim for <=40Mhz here. So yes, round up does make sense, but it
> seems to me that BXT_MAX_VAR_OUTPUT_KHZ should be 40 MHz.
> 
[Deepak, M] Yes, thought about it and me too feel that it should be 40 MHz.  We locally have tried with 2 different MIPI panels with 39.5 Mhz and didn't see any issue. I will confirm with SV teams and will update the patch accordingly. Thanks for pointing it :)

> >  	tmp |= BXT_MIPI_ESCLK_VAR_DIV(port, divider);
> >
> >  	/*
> > --
> > 1.9.1
> >
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> 
> --
> Ville Syrjälä
> Intel OTC