Re: [PATCH v2 2/2] pmbus: ltc2978: add regulator gating

From: Alan Tull
Date: Sun Aug 24 2014 - 09:30:27 EST

On Sat, Aug 23, 2014 at 10:10 AM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> On 08/22/2014 02:45 PM, Mark Brown wrote:
>> On Fri, Aug 22, 2014 at 04:11:34PM -0500, atull@xxxxxxxxxxxxxxxxxxxxx
>> wrote:
>>> From: Alan Tull <atull@xxxxxxxxxxxxxxxxxxxxx>
>>> Add regulator with support for enabling or disabling all
>>> supplies.
>> Reviwed-by: Mark Brown <broonie@xxxxxxxxxx>
>> though it still looks like you should be able to create generic
>> functions for the operations.
> Sorry I didn't have time to review the code myself. I'll have
> to check the datasheet about turning regulators on and off.
> Using page 0xff for the lm2978 looks wrong, as the chip supports
> up to 8 channels which should be controlled separately
> (I would assume) instead of turning them all on and off in
> one go. Maybe I am missing something, but my assumption would
> have been to have a separate regulator for each channel, and
> that each channel would have its own regulator which would be
> turned on and off separately. So I don't really understand the
> change between v1 and v2 of the core patch, which dropped the
> per-channel regulators. Someone will have to explain to me
> why that makes sense, especially since it means that I won't
> be able to use the regulator expansion in my system (which
> would require per-channel regulators, and which does not always
> have all channels enabled on a given chip).

The LTC2978 spec says that the OPERATION command register "responds to
the global page command (PAGE=0xFF)." I originally took it to mean
that it *only* responds to 0xFF. Now I get that yes I could turn
on/off each of 8 channels separately.

I will make the change have one regulator for each supply. It would
be useful to still have a global regulator for turning them all on/off
together if that is not completely odious.

> In respect to generic functions, that really depends on the scope
> of the regulators. As written, where all regulators are turned on
> in a single operation, per-chip functions are needed. I thought
> we would only need per-chip configuration values, but that only
> applies if regulators are turned on one by one, not all in one go.

For all the parts supported by ltc2978.c, a banked write of 0x80 to
the OPERATION register turns a regulator on, writing 0x00 turns it
off. I'm not sure how universal that is for other PMBUS parts. I
will look at the PMBUS specs when I get back to the office Tuesday.

I'll look into it and see if I can push almost all of this code into
pmbus_core.c and just have per-chip config values in
pmbus_driver_info. If many PMBUS parts have the same scheme (0x80 or
0x00 written to OPERATION register as a banked write) then a flag bit
that is turned on in pmbus_driver_info could enable this scheme for
this chip. If it is a different register or different on/off values,
then I'll need to add those to pmbus_driver_info.
> Either case, a wrapper for ltc2978_write_pmbus_operation needs to
> be added to pmbus_core.c as pmbus_write_byte_data and exported
> (as a separate patch). For paged reads, the existing
> pmbus_read_byte_data should be used. In general, avoid direct
> accesses to paged registers and use API functions instead
> if possible.

OK. Will add pmbus_write_byte_data as a separate patch and use the
existing pmbus_read_byte_data.

Thanks for the review!


> Thanks,
> Guenter
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