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: <CA+V-a8tXpWf8-YL-qzWhqc+fDvV4Kzd-6gJqC5HWvE00QsNFAA@mail.gmail.com>
Date: Wed, 23 Apr 2025 12:53:58 +0100
From: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
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 Geert,

Thank you for the review.

On Tue, Apr 22, 2025 at 8:41 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> 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)
>
OK.

> 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.
>
Agreed, I will update it to,
`DIV_ROUND_CLOSEST_ULL(mul_u32_u32(vclk_rate, bpp * KILO),
dsi->lanes);`

Cheers,
Prabhakar