linux-kernel - Re: [PATCH v3 10/15] drm: renesas: rz-du: mipi_dsi: Use mHz for D-PHY frequency calculations

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Message-ID: <CAMuHMdWs7R9rtS7Ur6PP9e3m9ghkM1jc_Xn3QOWG4rvTtB2omA@mail.gmail.com>
Date: Tue, 22 Apr 2025 09:41:07 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Prabhakar <prabhakar.csengg@...il.com>
Cc: 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>, Simona Vetter <simona@...ll.ch>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Biju Das <biju.das.jz@...renesas.com>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	Michael Turquette <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, 
	Philipp Zabel <p.zabel@...gutronix.de>, Magnus Damm <magnus.damm@...il.com>, 
	dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org, 
	linux-clk@...r.kernel.org, Fabrizio Castro <fabrizio.castro.jz@...esas.com>, 
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH v3 10/15] drm: renesas: rz-du: mipi_dsi: Use mHz for D-PHY
 frequency calculations

Hi Prabhakar,

On Fri, 18 Apr 2025 at 20:47, Prabhakar <prabhakar.csengg@...il.com> wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
>
> Pass the HSFREQ in milli-Hz to the `dphy_init()` callback to improve
> precision, especially for the RZ/V2H(P) SoC, where PLL dividers require
> high accuracy.
>
> These changes prepare the driver for upcoming RZ/V2H(P) SoC support.
>
> Co-developed-by: Fabrizio Castro <fabrizio.castro.jz@...esas.com>
> Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@...esas.com>
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> ---
> v2->v3:
> - Replaced `unsigned long long` with `u64`
> - Replaced *_mhz with *_millihz` in functions

Thanks for the update!

> @@ -203,8 +203,9 @@ static u32 rzg2l_mipi_dsi_link_read(struct rzg2l_mipi_dsi *dsi, u32 reg)
>   */
>
>  static int rzg2l_mipi_dsi_dphy_init(struct rzg2l_mipi_dsi *dsi,
> -                                   unsigned long hsfreq)
> +                                   u64 hsfreq_millihz)
>  {
> +       unsigned long hsfreq = DIV_ROUND_CLOSEST_ULL(hsfreq_millihz, KILO);

MILLI (everywhere)

It's a strange world where KILO == MILLI ;-)

    include/linux/units.h:#define KILO      1000UL
    include/linux/units.h-#define MILLI     1000UL

>         const struct rzg2l_mipi_dsi_timings *dphy_timings;
>         unsigned int i;
>         u32 dphyctrl0;
> @@ -277,6 +278,7 @@ static int rzg2l_mipi_dsi_startup(struct rzg2l_mipi_dsi *dsi,
>                                   const struct drm_display_mode *mode)
>  {
>         unsigned long hsfreq, vclk_rate;
> +       u64 hsfreq_millihz;
>         unsigned int bpp;
>         u32 txsetr;
>         u32 clstptsetr;
> @@ -305,9 +307,9 @@ static int rzg2l_mipi_dsi_startup(struct rzg2l_mipi_dsi *dsi,
>          */
>         bpp = mipi_dsi_pixel_format_to_bpp(dsi->format);
>         vclk_rate = clk_get_rate(dsi->vclk);
> -       hsfreq = DIV_ROUND_CLOSEST_ULL(vclk_rate * bpp, dsi->lanes);
> +       hsfreq_millihz = DIV_ROUND_CLOSEST_ULL(vclk_rate * bpp * KILO * 1ULL, dsi->lanes);

The "* 1ULL" only makes the last factor unsigned long long.
"vclk_rate * bpp" is still unsigned long, causing overflow on 32-bit.
As there is no rounding variant of mul_u64_u32_div(), you probably
want to use mul_u32_u32() instead.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds