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: 6197aa65c4 ("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>
e1f1ae8002