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

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Date:   Sun, 16 Apr 2023 17:31:24 -0500
From:   Adam Ford <aford173@...il.com>
To:     Marek Vasut <marex@...x.de>
Cc:     dri-devel@...ts.freedesktop.org, m.szyprowski@...sung.com,
        aford@...conembedded.com, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Inki Dae <inki.dae@...sung.com>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Frieder Schrempf <frieder.schrempf@...tron.de>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] drm: bridge: samsung-dsim: Fix PMS Calculator on imx8m[mnp]

On Sun, Apr 16, 2023 at 5:07 PM Marek Vasut <marex@...x.de> 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.

adam

> "