Leonard Crestez observed the following phenomenon: when using
hard interrupt triggers (the DRDY line coming out of an ST
sensor) sometimes a new value would arrive while reading the
previous value, due to latencies in the system.
We discovered that the ST hardware as far as can be observed
is designed for level interrupts: the DRDY line will be held
asserted as long as there are new values coming. The interrupt
handler should be re-entered until we're out of values to
handle from the sensor.
If interrupts were handled as occurring on the edges (usually
low-to-high) new values could appear and the line be held
asserted after that, and these values would be missed, the
interrupt handler would also lock up as new data was
available, but as no new edges occurs on the DRDY signal,
nothing happens: the edge detector only detects edges.
To counter this, do the following:
- Accept interrupt lines to be flagged as level interrupts
using IRQF_TRIGGER_HIGH and IRQF_TRIGGER_LOW. If the line
is marked like this (in the device tree node or ACPI
table or similar) it will be utilized as a level IRQ.
We mark the line with IRQF_ONESHOT and mask the IRQ
while processing a sample, then the top half will be
entered again if new values are available.
- If we are flagged as using edge interrupts with
IRQF_TRIGGER_RISING or IRQF_TRIGGER_FALLING: remove
IRQF_ONESHOT so that the interrupt line is not
masked while running the thread part of the interrupt.
This way we will never miss an interrupt, then introduce
a loop that polls the data ready registers repeatedly
until no new samples are available, then exit the
interrupt handler. This way we know no new values are
available when the interrupt handler exits and
new (edge) interrupts will be triggered when data arrives.
Take some extra care to update the timestamp in the poll
loop if this happens. The timestamp will not be 100%
perfect, but it will at least be closer to the actual
events. Usually the extra poll loop will handle the new
samples, but once in a blue moon, we get a new IRQ
while exiting the loop, before returning from the
thread IRQ bottom half with IRQ_HANDLED. On these rare
occasions, the removal of IRQF_ONESHOT means the
interrupt will immediately fire again.
- If no interrupt type is indicated from the DT/ACPI,
choose IRQF_TRIGGER_RISING as default, as this is necessary
for legacy boards.
Tested successfully on the LIS331DL and L3G4200D by setting
sampling frequency to 400Hz/800Hz and stressing the system:
extra reads in the threaded interrupt handler occurs.
Cc: Giuseppe Barba <giuseppe.barba@st.com>
Cc: Denis Ciocca <denis.ciocca@st.com>
Tested-by: Crestez Dan Leonard <cdleonard@gmail.com>
Reported-by: Crestez Dan Leonard <cdleonard@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
90efe05562