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Subject[PATCH] clk: renesas: cpg-mssr: Fix reset control race condition
Date
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: 6197aa65c4905532 ("clk: renesas: cpg-mssr: Add support for reset control")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
So far I haven't been able to reproduce the issue on R-Car Gen2 (after
forcing i2c reset on Gen2, too). Perhaps my Koelsch doesn't have enough
CPU cores. What about Lager?

Hi Mike, Stephen,

As this is a bugfix, can you please take this directly, if accepted?

Thanks!
---
drivers/clk/renesas/renesas-cpg-mssr.c | 16 ++--------------
1 file changed, 2 insertions(+), 14 deletions(-)

diff --git a/drivers/clk/renesas/renesas-cpg-mssr.c b/drivers/clk/renesas/renesas-cpg-mssr.c
index 52bbb9ce3807db31..d4075b13067429cd 100644
--- a/drivers/clk/renesas/renesas-cpg-mssr.c
+++ b/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;
}

--
2.17.1
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