linux-kernel - [PATCH] clk: ingenic-tcu: Properly enable registers before accessing timers

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Date:   Fri,  3 Jun 2022 14:47:05 +0100
From:   Aidan MacDonald <aidanmacdonald.0x0@...il.com>
To:     paul@...pouillou.net, mturquette@...libre.com, sboyd@...nel.org
Cc:     linux-mips@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] clk: ingenic-tcu: Properly enable registers before accessing timers

Access to registers is guarded by ingenic_tcu_{enable,disable}_regs()
so the stop bit can be cleared before accessing a timer channel, but
those functions did not clear the stop bit on SoCs with a global TCU
clock gate.

Testing on the X1000 has revealed that the stop bits must be cleared
_and_ the global TCU clock must be ungated to access timer registers.
Programming manuals for the X1000, JZ4740, and JZ4725B specify this
behavior. If the stop bit isn't cleared, then writes to registers do
not take effect, which can leave clocks with no defined parent when
registered and leave clock tree state out of sync with the hardware,
triggering bugs in downstream drivers relying on TCU clocks.

Fixing this is easy: have ingenic_tcu_{enable,disable}_regs() always
clear the stop bit, regardless of the presence of a global TCU gate.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@...il.com>
---
 drivers/clk/ingenic/tcu.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/drivers/clk/ingenic/tcu.c b/drivers/clk/ingenic/tcu.c
index 201bf6e6b6e0..d5544cbc5c48 100644
--- a/drivers/clk/ingenic/tcu.c
+++ b/drivers/clk/ingenic/tcu.c
@@ -101,15 +101,11 @@ static bool ingenic_tcu_enable_regs(struct clk_hw *hw)
 	bool enabled = false;

 	/*
-	 * If the SoC has no global TCU clock, we must ungate the channel's
-	 * clock to be able to access its registers.
-	 * If we have a TCU clock, it will be enabled automatically as it has
-	 * been attached to the regmap.
+	 * According to the programming manual, a timer channel's registers can
+	 * only be accessed when the channel's stop bit is clear.
 	 */
-	if (!tcu->clk) {
-		enabled = !!ingenic_tcu_is_enabled(hw);
-		regmap_write(tcu->map, TCU_REG_TSCR, BIT(info->gate_bit));
-	}
+	enabled = !!ingenic_tcu_is_enabled(hw);
+	regmap_write(tcu->map, TCU_REG_TSCR, BIT(info->gate_bit));

 	return enabled;
 }
@@ -120,8 +116,7 @@ static void ingenic_tcu_disable_regs(struct clk_hw *hw)
 	const struct ingenic_tcu_clk_info *info = tcu_clk->info;
 	struct ingenic_tcu *tcu = tcu_clk->tcu;

-	if (!tcu->clk)
-		regmap_write(tcu->map, TCU_REG_TSSR, BIT(info->gate_bit));
+	regmap_write(tcu->map, TCU_REG_TSSR, BIT(info->gate_bit));
 }

 static u8 ingenic_tcu_get_parent(struct clk_hw *hw)
-- 
2.35.1