Re: [PATCH 2/6] drm: bridge: samsung-dsim: Fix PMS Calculator on imx8m[mnp]

[Adam Ford]

On Mon, Apr 17, 2023 at 2:00 AM Alexander Stein
<alexander.stein@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> Am Montag, 17. April 2023, 00:31:24 CEST schrieb Adam Ford:
> > On Sun, Apr 16, 2023 at 5:07 PM Marek Vasut <marex@xxxxxxx> wrote:
> > > On 4/15/23 12:40, Adam Ford wrote:
> > > > According to Table 13-45 of the i.MX8M Mini Reference Manual, the min
> > > > and max values for M and  the frequency range for the VCO_out
> > > > calculator were incorrect.  This also appears to be the case for the
> > > > imx8mn and imx8mp.
> > > >
> > > > To fix this, make new variables to hold the min and max values of m
> > > > and the minimum value of VCO_out, and update the PMS calculator to
> > > > use these new variables instead of using hard-coded values to keep
> > > > the backwards compatibility with other parts using this driver.
> > >
> > > [...]
> > >
> > > >   static const struct samsung_dsim_driver_data imx8mm_dsi_driver_data =
> > > >   {
> > > >
> > > > @@ -470,6 +485,9 @@ static const struct samsung_dsim_driver_data
> > > > imx8mm_dsi_driver_data = {> >
> > > >        */
> > > >
> > > >       .pll_p_offset = 14,
> > > >       .reg_values = imx8mm_dsim_reg_values,
> > > >
> > > > +     .m_min = 64,
> > > > +     .m_max = 1023,
> > > > +     .vco_min = 1050,
> > >
> > > You might want to call this 'min_freq' since there is a 'max_freq' which
> > > seems to indicate what VCO max frequency is.
> > >
> > > Note that the same datasheet contains the following information:
> > > "
> > > MIPI_DPHY_M_PLLPMS field descriptions
> > >
> > > 12–4 PMS_M
> > > Specifies the PLL PMS value for the M divider
> > > NOTE: The programmable divider range should be within 25 to 125 to
> > > ensure PLL stability.
> >
> > I was confused by this because this statement is not consistent with
> > the link they reference jumps me to the table where it reads M is
> > between 64 and 1023.
> >
> > > NOTE: The M and P divider values should be considered together to ensure
> > > VCO ouput frequency
> > > (VCO_out) range is between 350 MHz to 750 MHz.
> > > Please refer to the topic DPHY PLL for more information.
> >
> > I was confused by this too, because the NXP documentation reads the
> > 350 - 750MHz that you state, but  "Table 13-45: DPHY PLL Parameters"
> > which immediately follows that sentence  on page 4158 shows VCO_out is
> > between 1050-2100 MHz.
> >
> > I compared the PMS values for a variety of frequencies to those that
> > were set in the downstream NXP code, and the PMS values matched.
> > Maybe someone from NXP can explain the discrepancy.
>
> Also note that, according to Table 13-28 in RM (Rev 2 07/2022) for i.MX8M
> Nano, VCO_out and Fout has a maximum of 1500MHz only. Although the note above
> mentions a range  of 1050-2100MHz...

I looked up the limits in NXP's downstream kernel [1] , and I believe
these values match the table in the reference manual instead of the
conflicting sentence.  I am guessing this is why the PMS values I get
match those of the NXP downstream kernel.

adam

[1] - https://github.com/nxp-imx/linux-imx/blob/lf-6.1.y/drivers/gpu/drm/imx/sec_mipi_pll_1432x.h#L38

>
> Best regards,
> Alexander
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