Adds a section covering CONFIG_SENSORS_LSM6DS3TRC_FIFO: what it changes
(one interrupt/I2C read per watermark instead of per sample) and its
three limitations while it's on -- both sub-sensors forced to the same
ODR, temperature no longer per-sample (one read per drain applied to
the whole batch), and both sub-sensors must stay physically enabled
regardless of subscription, since the chip's FIFO write trigger needs
both running -- plus the watermark/ORB-buffer-size relationship callers
need to respect.
Also documents a related chip quirk found during bench testing:
diff_words reads 0 at the exact moment a real FIFO overrun occurs, even
though the FIFO is still completely full of valid, retained data (a
forced read past that point recovers real samples, not garbage). This
isn't a bug in this driver: ST's own engineers confirm the same
behavior for this chip family on their community forum (thread
"LSM6DS3 FIFO status clarification", td-p/184022), and the mainline
Linux st_lsm6dsx driver doesn't attempt to recover from it either -- it
only special-cases an empty FIFO, not an overrun one. Documenting this
in a comment rather than adding recovery logic: with the small
watermark this driver uses, reaching a real overrun at all means the
drain has already fallen many seconds behind, and ST's own guidance for
this condition is to avoid it via watermark sizing rather than recover
from it.
Signed-off-by: Felipe Moura <moura.fmo@gmail.com>
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