linux-kernel - [PATCH 1/3] clk: meson: mpll: properly handle spread spectrum

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Message-Id: <20190329153325.23869-2-jbrunet@baylibre.com>
Date:   Fri, 29 Mar 2019 16:33:23 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     Jerome Brunet <jbrunet@...libre.com>,
        linux-amlogic@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] clk: meson: mpll: properly handle spread spectrum

The bit 'SSEN' available on some MPLL DSS outputs is not related to the
fractional part of the divider but to the function called
'Spread Spectrum'.

This function might be used to solve EM issues by adding a jitter on
clock signal. This widens the signal spectrum and weakens the peaks in it.

While spread spectrum might be useful for some application, it is
problematic for others, such as audio.

This patch introduce a new flag to the MPLL driver to enable (or not) the
spread spectrum function.

Fixes: 1f737ffa13ef ("clk: meson: mpll: fix mpll0 fractional part ignored")
Signed-off-by: Jerome Brunet <jbrunet@...libre.com>
---
 drivers/clk/meson/clk-mpll.c | 9 ++++++---
 drivers/clk/meson/clk-mpll.h | 1 +
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/clk/meson/clk-mpll.c b/drivers/clk/meson/clk-mpll.c
index 64d31c8ba3d0..2d39a8bc367c 100644
--- a/drivers/clk/meson/clk-mpll.c
+++ b/drivers/clk/meson/clk-mpll.c
@@ -141,9 +141,12 @@ static void mpll_init(struct clk_hw *hw)
 	/* Enable the fractional part */
 	meson_parm_write(clk->map, &mpll->sdm_en, 1);
 
-	/* Set additional fractional part enable if required */
-	if (MESON_PARM_APPLICABLE(&mpll->ssen))
-		meson_parm_write(clk->map, &mpll->ssen, 1);
+	/* Set spread spectrum if possible */
+	if (MESON_PARM_APPLICABLE(&mpll->ssen)) {
+		unsigned int ss =
+			mpll->flags & CLK_MESON_MPLL_SPREAD_SPECTRUM ? 1 : 0;
+		meson_parm_write(clk->map, &mpll->ssen, ss);
+	}
 
 	/* Set the magic misc bit if required */
 	if (MESON_PARM_APPLICABLE(&mpll->misc))
diff --git a/drivers/clk/meson/clk-mpll.h b/drivers/clk/meson/clk-mpll.h
index 2925fb939fdd..a991d568c43a 100644
--- a/drivers/clk/meson/clk-mpll.h
+++ b/drivers/clk/meson/clk-mpll.h
@@ -25,6 +25,7 @@ struct meson_clk_mpll_data {
 };
 
 #define CLK_MESON_MPLL_ROUND_CLOSEST	BIT(0)
+#define CLK_MESON_MPLL_SPREAD_SPECTRUM	BIT(1)
 
 extern const struct clk_ops meson_clk_mpll_ro_ops;
 extern const struct clk_ops meson_clk_mpll_ops;
-- 
2.20.1