The existing scan code is based on sending an i2c START condition and if
there is no error it assumes there is a device at that i2c address. Some
i2c controllers don't support sending individual start/stop signals on the
bus, they can only perform complete data transfers with start/stop handled
in the silicon.
This adds a fallback mechanism that attempts to read a single byte from each
i2c address. It's less reliable than looking for an an ACK repsonse to a
start, because some devices will NAK an attempt to read that isn't preceeded
by a write of a register address. Writing to devices to probe them is too
dangerous to even consider. The user is told that a less-reliable scan is
being done, so even if the read-scan comes up empty too, it's still a vast
improvement over the old situation where it would just claim there were no
devices on the bus even though the devices were there and working fine.
If the i2c controller responds with a proper ENODEV (device doesn't support
operation) or an almost-proper EOPNOTSUPP, the START/STOP scan is switched
to a read-scan right away. Most controllers respond with ENXIO or EIO if
they don't support START/STOP, so no quick-out is available. For those,
if a scan of all 127 addresses and come up empty, the scan is re-done using
the read method.
Reported by: Maxim Filimonov <che@bein.link>
Some more automated I2C controllers cannot explicitly create
START/STOP/etc. conditions on the bus.
Instead, the correct condition is set automatically according
to the pending transfer status.
This particular behavior can cause trouble if some I2C slave
requires sending address offset within the chip followed by
the actual data or command. In that case we cannot assume that
the driver will not STOP immediately after sending
offset.
To avoid that, do not split offset transfer from data transfer
for default transmission modes and do exactly that if requested
in command line (stop-start and repeated-start modes).
This more generic approach should cover special cases like
the one described.
Reviewed by: imp
Submitted by: Marcin Mazurek <mma@semihalf.com>
Obtained from: Semihalf
Off by default, build behaves normally.
WITH_META_MODE we get auto objdir creation, the ability to
start build from anywhere in the tree.
Still need to add real targets under targets/ to build packages.
Differential Revision: D2796
Reviewed by: brooks imp
