linux-kernel - [PATCH] drm/msm/dsi/phy_7nm: fix rounding error in recalc

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Message-Id: <20251125-msm-dsi-phy-7nm-clk-rate-v1-1-17141806e3a0@oss.qualcomm.com>
Date: Tue, 25 Nov 2025 20:55:07 +0530
From: Prahlad Valluru <venkata.valluru@....qualcomm.com>
To: Rob Clark <robin.clark@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jesszhan0024@...il.com>, Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Prahlad Valluru <venkata.valluru@....qualcomm.com>
Subject: [PATCH] drm/msm/dsi/phy_7nm: fix rounding error in recalc_rate

Required vco rate is set by programming decimal and fraction
from 64 bit calculation. This programmed rate is not exactly
matching the requested rate and corresponding recalc_rate is
having rounding error due to this delta.

When setting byte_clk and byte_intf_clk from this pll,
set_rate on byte_intf_clk resulting in dividers getting
reprogrammed, which are already set from byte_clk.
Convert this recalc_rate to KHz and back to Hz to round up
this delta in calculation.

Signed-off-by: Prahlad Valluru <venkata.valluru@....qualcomm.com>
---
 drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c b/drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c
index 32f06edd21a9..00f20c5a7c73 100644
--- a/drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c
+++ b/drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c
@@ -598,6 +598,12 @@ static unsigned long dsi_pll_7nm_vco_recalc_rate(struct clk_hw *hw,
 	pll_freq += div_u64(tmp64, multiplier);
 
 	vco_rate = pll_freq;
+	/*
+	 * Recalculating the rate from dec and frac doesn't end up the rate
+	 * we originally set. Convert the freq to KHz, round it up and
+	 * convert it back to Hz.
+	 */
+	vco_rate = DIV_ROUND_UP_ULL(vco_rate, 1000) * 1000;
 	pll_7nm->vco_current_rate = vco_rate;
 
 	DBG("DSI PLL%d returning vco rate = %lu, dec = %x, frac = %x",

---
base-commit: 63c971af40365ee706c7e24f6a7900d693518f09
change-id: 20251125-msm-dsi-phy-7nm-clk-rate-26723a3b686c

Best regards,
-- 
Prahlad Valluru <venkata.valluru@....qualcomm.com>