hwmon: Add driver for fsp-3y PSUs and PDUs

This patch adds support for these devices:
- YH-5151E - the PDU
- YM-2151E - the PSU

The device datasheet says that the devices support PMBus 1.2, but in my
testing, a lot of the commands aren't supported and if they are, they
sometimes behave strangely or inconsistently. For example, writes to the
PAGE command requires using PEC, otherwise the write won't work and the
page won't switch, even though, the standard says that PEC is optional.
On the other hand, writes to SMBALERT don't require PEC. Because of
this, the driver is mostly reverse engineered with the help of a tool
called pmbus_peek written by David Brownell (and later adopted by my
colleague Jan Kundrát).

The device also has some sort of a timing issue when switching pages,
which is explained further in the code.

Because of this, the driver support is limited. It exposes only the
values that have been tested to work correctly.

Signed-off-by: Václav Kubernát <kubernat@cesnet.cz>
Link: https://lore.kernel.org/r/20210414080019.3530794-1-kubernat@cesnet.cz
[groeck: Fixed up "missing braces around initializer" from 0-day]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

1 files changed, 1 insertions, 0 deletions

diff --git a/Documentation/hwmon/index.rst b/Documentation/hwmon/index.rst
index a846eff10edf..d6a050f85477 100644
--- a/Documentation/hwmon/index.rst
+++ b/Documentation/hwmon/index.rst
@@ -63,6 +63,7 @@ Hardware Monitoring Kernel Drivers
    f71805f
    f71882fg
    fam15h_power
+   fsp-3y
    ftsteutates
    g760a
    g762