Re: [PATCH 2/3] hwmon: (pmbus/core) add POWER_GOOD signal limits support
From: Guenter Roeck
Date: Tue Sep 10 2024 - 12:23:08 EST
On 9/10/24 08:00, Jerome Brunet wrote:
On Tue 10 Sep 2024 at 07:37, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
On 9/9/24 23:43, Jerome Brunet wrote:
On Mon 09 Sep 2024 at 11:16, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
On 9/9/24 08:39, Jerome Brunet wrote:
Add support for POWER_GOOD_ON and POWER_GOOD_OFF standard PMBus commands.
For PMBus devices that offer a POWER_GOOD signal, these commands are used
for setting the output voltage at which a power good signal should be
asserted and negated.
Power Good signals are device and manufacturer specific. Many factors
other
than output voltage may be used to determine whether or not the POWER_GOOD
signal is to be asserted. PMBus device users are instructed to consult the
device manufacturer’s product literature for the specifics of the device
they are using.
Note that depending on the choice of the device manufacturer that a
device
may drive a POWER_GOOD signal high or low to indicate that the signal is
asserted.
Signed-off-by: Jerome Brunet <jbrunet@xxxxxxxxxxxx>
---
drivers/hwmon/pmbus/pmbus.h | 3 +++
drivers/hwmon/pmbus/pmbus_core.c | 6 ++++++
2 files changed, 9 insertions(+)
diff --git a/drivers/hwmon/pmbus/pmbus.h b/drivers/hwmon/pmbus/pmbus.h
index 5d5dc774187b..e322d2dd9fb7 100644
--- a/drivers/hwmon/pmbus/pmbus.h
+++ b/drivers/hwmon/pmbus/pmbus.h
@@ -78,6 +78,9 @@ enum pmbus_regs {
PMBUS_IIN_OC_FAULT_LIMIT = 0x5B,
PMBUS_IIN_OC_WARN_LIMIT = 0x5D,
+ PMBUS_POWER_GOOD_ON = 0x5E,
+ PMBUS_POWER_GOOD_OFF = 0x5F,
+
PMBUS_POUT_OP_FAULT_LIMIT = 0x68,
PMBUS_POUT_OP_WARN_LIMIT = 0x6A,
PMBUS_PIN_OP_WARN_LIMIT = 0x6B,
diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
index 0ea6fe7eb17c..94ddf0166770 100644
--- a/drivers/hwmon/pmbus/pmbus_core.c
+++ b/drivers/hwmon/pmbus/pmbus_core.c
@@ -1768,6 +1768,12 @@ static const struct pmbus_limit_attr vout_limit_attrs[] = {
.attr = "crit",
.alarm = "crit_alarm",
.sbit = PB_VOLTAGE_OV_FAULT,
+ }, {
+ .reg = PMBUS_POWER_GOOD_ON,
+ .attr = "good_on",
+ }, {
+ .reg = PMBUS_POWER_GOOD_OFF,
+ .attr = "good_off",
}, {
.reg = PMBUS_VIRT_READ_VOUT_AVG,
.update = true,
Those attributes are not hardware monitoring attributes and therefore not
acceptable. In general I am not sure if they should be configurable in the
first place, but definitely not from the hardware monitoring subsystem.
Maybe the regulator subsystem callbacks set_over_voltage_protection and
set_under_voltage_protection would be appropriate (with severity
REGULATOR_SEVERITY_PROT), but that should be discussed with regulator
subsystem maintainers.
According to PMBUS spec, there is no protection associated with that
command. It just tells when the output voltage is considered good, when
it is not. What it does after that really depends the device, it may
drive a pin for example (or an LED indicator in my case).
It is much more likely that it connects to the reset signal on the board,
or it enables/disables power to parts of the board.
That's not what PMBus spec says about it:
"""
15.32. POWER_GOOD Signal Limits
For PMBus devices that offer a POWER_GOOD signal, these commands are used for
setting the output voltage at which a power good signal should be asserted and negated.
Power Good signals will be device and manufacturer specific. Many factors other than
output voltage may be used to determine whether or not the POWER_GOOD signal is to
be asserted. PMBus device users are instructed to consult the device manufacturer’s
product literature for the specifics of the device they are using.
"""
It's only supposed to have an effect on the power_good signal, not the
reset. I guess someone could wire that signal to a reset. Same could be
done with the alert or the fault one, I suppose
It doesn't say anything about the _use_ of that signal. The PMBus specification
says "Power Good signals will be device and manufacturer specific", and that
is exactly what it is. TPS25990 specifically states that the signal indicates
that it is ok for downstream chips to draw power, which is a very typical use.
The ability to connect it it to an LED does not reflect its core use.
It is very similar to 'crit' or other limits in that sense,
I think. I don't really get why such property is not OK in hwmon then
and why it should not be configurable, if the other limits are ?
Its use is for hardware control, not monitoring, even if it may be connected
to a status LED. MAX15301, for example, groups the command under "Voltage
Sequencing Commands".
On top of that, the voltages are value/hysteresis values. The "off" voltage
is lower than the "on" voltage.
TPS25990 doesn't even support the command according to its datasheet, so I am
at loss about your use case in the context of this patch series (the PGOOD pin
on this chip signals to the downstream load that it is ok to draw
power).
It does support GOOD_OFF, althought TI renamed the register to
VOUT_PGTH (Section 8.3.14.7.1.52, p87):
"""
VOUT_PGTH is a standard PMBus® command for setting or reading an 8-bit
output voltage threshold at which Power Good (PGOOD) is be de-asserted.
"""
Ah yes, typical for PMBus chips :-(. Why use standard register/command names
if one can rename them. It actually also states that pgood is asserted first
when the voltage reaches VOUT_PGTH + 250mV, so even with this chip it is
really a hysteresis.
Same as the PMBus spec. Changing the value through this command does
affect the signal as intented. How the signal is depends on the
implementation. It just drives an LED on the EVM.
Yes, but that doesn't make it a hardware _monitoring_ attribute.
Guenter