spmi: spmi-pmic-arb: fix irq_set_type race condition

The qpnpint_irq_set_type() callback function configures the type
(edge vs level) and polarity (high, low, or both) of a particular
PMIC interrupt within a given peripheral.  To do this, it reads
the three consecutive IRQ configuration registers, modifies the
specified IRQ bit within the register values, and finally writes
the three modified register values back to the PMIC.  While a
spinlock is used to provide mutual exclusion on the SPMI bus
during the register read and write calls, there is no locking
around the overall read, modify, write sequence.  This opens up
the possibility of a race condition if two tasks set the type of
a PMIC IRQ within the same peripheral simultaneously.

When the race condition is encountered, both tasks will read the
old value of the registers and IRQ bits set by one of the tasks
will be dropped upon the register write of the other task.  This
then leads to PMIC IRQs being enabled with an incorrect type and
polarity configured.  Such misconfiguration can lead to an IRQ
storm that overwhelms the system and causes it to crash.

This race condition and IRQ storm have been observed when using
a pair of pm8941-pwrkey devices to handle PMK8350 pwrkey and
resin interrupts.  The independent devices probe asynchronously
in parallel and can simultaneously request and configure PMIC
IRQs in the same PMIC peripheral.

For a good case, the IRQ configuration calls end up serialized
due to timing deltas and the register read/write sequence looks
like this:

1. pwrkey probe: SPMI  read(0x1311): 0x00, 0x00, 0x00
2. pwrkey probe: SPMI write(0x1311): 0x80, 0x80, 0x80
3. resin probe:  SPMI  read(0x1311): 0x80, 0x80, 0x80
4. resin probe:  SPMI write(0x1311): 0xC0, 0xC0, 0xC0

The final register states after both devices have requested and
enabled their respective IRQs is thus:

0x1311: 0xC0
0x1312: 0xC0
0x1313: 0xC0
0x1314: 0x00
0x1315: 0xC0

For a bad case, the IRQ configuration calls end up occurring
simultaneously and the race condition is encountered.  The
register read/write sequence then looks like this:

1. pwrkey probe: SPMI  read(0x1311): 0x00, 0x00, 0x00
2. resin probe:  SPMI  read(0x1311): 0x00, 0x00, 0x00
3. pwrkey probe: SPMI write(0x1311): 0x80, 0x80, 0x80
4. resin probe:  SPMI write(0x1311): 0x40, 0x40, 0x40

In this case, the final register states after both devices have
requested and enabled their respective IRQs is thus:

0x1311: 0x40
0x1312: 0x40
0x1313: 0x40
0x1314: 0x00
0x1315: 0xC0

This corresponds to the resin IRQ being configured for both
rising and falling edges, as expected.  However, the pwrkey IRQ
is misconfigured as level type with both polarity high and low
set to disabled.  The PMIC IRQ triggering hardware treats this
particular register configuration as if level low triggering is
enabled.

The raw pwrkey IRQ signal is low when the power key is not being
pressed.  Thus, the pwrkey IRQ begins firing continuously in an
IRQ storm.

Fix the race condition by holding the spmi-pmic-arb spinlock for
the duration of the read, modify, write sequence performed in the
qpnpint_irq_set_type() function.  Split the pmic_arb_read_cmd()
and pmic_arb_write_cmd() functions each into three parts so that
hardware register IO is decoupled from spinlock locking.  This
allows a new function pmic_arb_masked_write() to be added which
locks the spinlock and then calls register IO functions to
perform SPMI read and write commands in a single atomic
operation.

Signed-off-by: David Collins <quic_collinsd@quicinc.com>
Link: https://lore.kernel.org/r/20211118034719.28971-1-quic_collinsd@quicinc.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
1 file changed