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Message-ID: <20260123152235.GJ215800@killaraus>
Date: Fri, 23 Jan 2026 17:22:35 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Marek Vasut <marek.vasut+renesas@...lbox.org>
Cc: dri-devel@...ts.freedesktop.org, kernel test robot <lkp@...el.com>,
David Airlie <airlied@...il.com>,
Geert Uytterhoeven <geert+renesas@...der.be>,
Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Magnus Damm <magnus.damm@...il.com>,
Maxime Ripard <mripard@...nel.org>, Simona Vetter <simona@...ll.ch>,
Thomas Zimmermann <tzimmermann@...e.de>,
Tomi Valkeinen <tomi.valkeinen+renesas@...asonboard.com>,
linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH v2] drm/rcar-du: dsi: Clean up VCLK divider calculation
Hi Marek,
Thank you for the patch.
On Mon, Jan 05, 2026 at 06:52:04PM +0100, Marek Vasut wrote:
> Currently, in rcar_mipi_dsi_parameters_calc(), the VCLK divider is stored
> in setup_info structure as BIT(divider). The rcar_mipi_dsi_parameters_calc()
> is called at the early beginning of rcar_mipi_dsi_startup() function. Later,
> in the same rcar_mipi_dsi_startup() function, the stored BIT(divider) value
> is passed to __ffs() to calculate back the divider out of the value again.
>
> Factor out VCLK divider calculation into rcar_mipi_dsi_vclk_divider()
> function and call the function from both rcar_mipi_dsi_parameters_calc()
> and rcar_mipi_dsi_startup() to avoid this back and forth BIT() and _ffs()
> and avoid unnecessarily storing the divider value in setup_info at all.
>
> This rework has a slight side-effect, in that it should allow the compiler
> to better evaluate the code and avoid compiler warnings about variable
> value overflows, which can never happen.
>
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202512051834.bESvhDiG-lkp@intel.com/
> Closes: https://lore.kernel.org/oe-kbuild-all/202512222321.TeY4VbvK-lkp@intel.com/
> Signed-off-by: Marek Vasut <marek.vasut+renesas@...lbox.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
Tomi, could you please push this to drm-misc ?
> ---
> Cc: David Airlie <airlied@...il.com>
> Cc: Geert Uytterhoeven <geert+renesas@...der.be>
> Cc: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
> Cc: Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> Cc: Magnus Damm <magnus.damm@...il.com>
> Cc: Maxime Ripard <mripard@...nel.org>
> Cc: Simona Vetter <simona@...ll.ch>
> Cc: Thomas Zimmermann <tzimmermann@...e.de>
> Cc: Tomi Valkeinen <tomi.valkeinen+renesas@...asonboard.com>
> Cc: dri-devel@...ts.freedesktop.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-renesas-soc@...r.kernel.org
> ---
> V2: Use BIT_U16()
> ---
> .../gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c | 35 ++++++++++++++-----
> 1 file changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c b/drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c
> index 4ef2e3c129ed7..508977b9e8926 100644
> --- a/drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c
> +++ b/drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c
> @@ -84,7 +84,6 @@ struct dsi_setup_info {
> unsigned long fout;
> u16 m;
> u16 n;
> - u16 vclk_divider;
> const struct dsi_clk_config *clkset;
> };
>
> @@ -335,10 +334,24 @@ rcar_mipi_dsi_post_init_phtw_v4h(struct rcar_mipi_dsi *dsi,
> * Hardware Setup
> */
>
> +static unsigned int rcar_mipi_dsi_vclk_divider(struct rcar_mipi_dsi *dsi,
> + struct dsi_setup_info *setup_info)
> +{
> + switch (dsi->info->model) {
> + case RCAR_DSI_V3U:
> + default:
> + return (setup_info->clkset->vco_cntrl >> 4) & 0x3;
> +
> + case RCAR_DSI_V4H:
> + return (setup_info->clkset->vco_cntrl >> 3) & 0x7;
> + }
> +}
> +
> static void rcar_mipi_dsi_pll_calc(struct rcar_mipi_dsi *dsi,
> unsigned long fin_rate,
> unsigned long fout_target,
> - struct dsi_setup_info *setup_info)
> + struct dsi_setup_info *setup_info,
> + u16 vclk_divider)
> {
> unsigned int best_err = -1;
> const struct rcar_mipi_dsi_device_info *info = dsi->info;
> @@ -360,7 +373,7 @@ static void rcar_mipi_dsi_pll_calc(struct rcar_mipi_dsi *dsi,
> if (fout < info->fout_min || fout > info->fout_max)
> continue;
>
> - fout = div64_u64(fout, setup_info->vclk_divider);
> + fout = div64_u64(fout, vclk_divider);
>
> if (fout < setup_info->clkset->min_freq ||
> fout > setup_info->clkset->max_freq)
> @@ -390,7 +403,9 @@ static void rcar_mipi_dsi_parameters_calc(struct rcar_mipi_dsi *dsi,
> unsigned long fout_target;
> unsigned long fin_rate;
> unsigned int i;
> + unsigned int div;
> unsigned int err;
> + u16 vclk_divider;
>
> /*
> * Calculate Fout = dot clock * ColorDepth / (2 * Lane Count)
> @@ -412,18 +427,20 @@ static void rcar_mipi_dsi_parameters_calc(struct rcar_mipi_dsi *dsi,
>
> fin_rate = clk_get_rate(clk);
>
> + div = rcar_mipi_dsi_vclk_divider(dsi, setup_info);
> +
> switch (dsi->info->model) {
> case RCAR_DSI_V3U:
> default:
> - setup_info->vclk_divider = 1 << ((clk_cfg->vco_cntrl >> 4) & 0x3);
> + vclk_divider = BIT_U16(div);
> break;
>
> case RCAR_DSI_V4H:
> - setup_info->vclk_divider = 1 << (((clk_cfg->vco_cntrl >> 3) & 0x7) + 1);
> + vclk_divider = BIT_U16(div + 1);
> break;
> }
>
> - rcar_mipi_dsi_pll_calc(dsi, fin_rate, fout_target, setup_info);
> + rcar_mipi_dsi_pll_calc(dsi, fin_rate, fout_target, setup_info, vclk_divider);
>
> /* Find hsfreqrange */
> setup_info->hsfreq = setup_info->fout * 2;
> @@ -439,7 +456,7 @@ static void rcar_mipi_dsi_parameters_calc(struct rcar_mipi_dsi *dsi,
> dev_dbg(dsi->dev,
> "Fout = %u * %lu / (%u * %u * %u) = %lu (target %lu Hz, error %d.%02u%%)\n",
> setup_info->m, fin_rate, dsi->info->n_mul, setup_info->n,
> - setup_info->vclk_divider, setup_info->fout, fout_target,
> + vclk_divider, setup_info->fout, fout_target,
> err / 100, err % 100);
>
> dev_dbg(dsi->dev,
> @@ -653,11 +670,11 @@ static int rcar_mipi_dsi_startup(struct rcar_mipi_dsi *dsi,
> switch (dsi->info->model) {
> case RCAR_DSI_V3U:
> default:
> - vclkset |= VCLKSET_DIV_V3U(__ffs(setup_info.vclk_divider));
> + vclkset |= VCLKSET_DIV_V3U(rcar_mipi_dsi_vclk_divider(dsi, &setup_info));
> break;
>
> case RCAR_DSI_V4H:
> - vclkset |= VCLKSET_DIV_V4H(__ffs(setup_info.vclk_divider) - 1);
> + vclkset |= VCLKSET_DIV_V4H(rcar_mipi_dsi_vclk_divider(dsi, &setup_info));
> break;
> }
>
--
Regards,
Laurent Pinchart
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