For Elan touchscreen, https://chromium-review.googlesource.com/915583 has already solved it.  However, there are still some unimplemented useful functions for:
* eMMC bus.
* Display panel which can be identified by EDID values.
* Some I2C devices.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/factory/+/6560a09e282729ca561f23a0d596b21c616bec95

commit 6560a09e282729ca561f23a0d596b21c616bec95
Author: Yong Hong <yhong@chromium.org>
Date: Tue Mar 20 19:16:05 2018

probe: Implement the probe function for eMMC devices.

This CL implements the probe function for devices communicated
with eMMC bus.

BUG=chromium:807180
TEST=make test; manually test on DUT.

Change-Id: I59698258f57996effeea7257f2067c5ed5a8442d
Reviewed-on: https://chromium-review.googlesource.com/968281
Commit-Ready: Yong Hong <yhong@google.com>
Tested-by: Yong Hong <yhong@google.com>
Reviewed-by: Yilun Lin <yllin@chromium.org>

[add] https://crrev.com/6560a09e282729ca561f23a0d596b21c616bec95/py/probe/functions/mmc.py
[add] https://crrev.com/6560a09e282729ca561f23a0d596b21c616bec95/py/probe/functions/mmc_unittest.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/factory/+/78391ed5d372acbbc4240fa67cabae2961b66bbd

commit 78391ed5d372acbbc4240fa67cabae2961b66bbd
Author: Yong Hong <yhong@chromium.org>
Date: Thu Mar 29 17:25:24 2018

probe: Revise `edid` function to cover more general case.

This CL lets the `edid` function search edid files in more directory in
sysfs to prevent the function from failing to probe.  This CL also
implements the caching mechanism for the `edid` function.

BUG=chromium:807180
TEST=make test; run `probe eval-function edid` on DUT.

Change-Id: I3007aeacb76c7d2269f909d3b36abc5d118dc8b3
Reviewed-on: https://chromium-review.googlesource.com/974625
Commit-Ready: Yong Hong <yhong@google.com>
Tested-by: Yong Hong <yhong@google.com>
Reviewed-by: Yilun Lin <yllin@chromium.org>

[modify] https://crrev.com/78391ed5d372acbbc4240fa67cabae2961b66bbd/py/probe/functions/edid_unittest.py
[modify] https://crrev.com/78391ed5d372acbbc4240fa67cabae2961b66bbd/py/probe/functions/edid.py