clk: renesas: cpg-mssr: Fix reset control race condition

The module reset code in the Renesas CPG/MSSR driver uses
read-modify-write (RMW) operations to write to a Software Reset Register
(SRCRn), and simple writes to write to a Software Reset Clearing
Register (SRSTCLRn), as was mandated by the R-Car Gen2 and Gen3 Hardware
User's Manuals.

However, this may cause a race condition when two devices are reset in
parallel: if the reset for device A completes in the middle of the RMW
operation for device B, device A may be reset again, causing subtle
failures (e.g. i2c timeouts):

	thread A			thread B
	--------			--------

	val = SRCRn
	val |= bit A
	SRCRn = val

	delay

					val = SRCRn (bit A is set)

	SRSTCLRn = bit A
	(bit A in SRCRn is cleared)

					val |= bit B
					SRCRn = val (bit A and B are set)

This can be reproduced on e.g. Salvator-XS using:

    $ while true; do i2cdump -f -y 4 0x6A b > /dev/null; done &
    $ while true; do i2cdump -f -y 2 0x10 b > /dev/null; done &

    i2c-rcar e6510000.i2c: error -110 : 40000002
    i2c-rcar e66d8000.i2c: error -110 : 40000002

According to the R-Car Gen3 Hardware Manual Errata for Rev.
0.80 of Feb 28, 2018, reflected in Rev. 1.00 of the R-Car Gen3 Hardware
User's Manual, writes to SRCRn do not require read-modify-write cycles.

Note that the R-Car Gen2 Hardware User's Manual has not been updated
yet, and still says a read-modify-write sequence is required.  According
to the hardware team, the reset hardware block is the same on both R-Car
Gen2 and Gen3, though.

Hence fix the issue by replacing the read-modify-write operations on
SRCRn by simple writes.
Reported-by: Yao Lihua <Lihua.Yao@desay-svautomotive.com>
Fixes: 6197aa65 ("clk: renesas: cpg-mssr: Add support for reset control")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Linh Phung <linh.phung.jy@renesas.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/renesas/renesas-cpg-mssr.c

@@ -572,17 +572,11 @@ static int cpg_mssr_reset(struct reset_controller_dev *rcdev,
 	unsigned int reg = id / 32;
 	unsigned int bit = id % 32;
 	u32 bitmask = BIT(bit);
-	unsigned long flags;
-	u32 value;
 
 	dev_dbg(priv->dev, "reset %u%02u\n", reg, bit);
 
 	/* Reset module */
-	spin_lock_irqsave(&priv->rmw_lock, flags);
-	value = readl(priv->base + SRCR(reg));
-	value |= bitmask;
-	writel(value, priv->base + SRCR(reg));
-	spin_unlock_irqrestore(&priv->rmw_lock, flags);
+	writel(bitmask, priv->base + SRCR(reg));
 
 	/* Wait for at least one cycle of the RCLK clock (@ ca. 32 kHz) */
 	udelay(35);
@@ -599,16 +593,10 @@ static int cpg_mssr_assert(struct reset_controller_dev *rcdev, unsigned long id)
 	unsigned int reg = id / 32;
 	unsigned int bit = id % 32;
 	u32 bitmask = BIT(bit);
-	unsigned long flags;
-	u32 value;
 
 	dev_dbg(priv->dev, "assert %u%02u\n", reg, bit);
 
-	spin_lock_irqsave(&priv->rmw_lock, flags);
-	value = readl(priv->base + SRCR(reg));
-	value |= bitmask;
-	writel(value, priv->base + SRCR(reg));
-	spin_unlock_irqrestore(&priv->rmw_lock, flags);
+	writel(bitmask, priv->base + SRCR(reg));
 	return 0;
 }