iio: st_sensors: harden interrupt handling
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:
Linus Walleij <linus.walleij@linaro.org> Signed-off-by:
Jonathan Cameron <jic23@kernel.org>
Showing