[PATCH] ARM: dts: am335x-bone* enable pmic-shutdown-controller

[Robert Nelson]

On Mon, May 18, 2015 at 10:21 AM, Tony Lindgren <tony at atomide.com> wrote:
> * Matthijs van Duin <matthijsvanduin at gmail.com> [150515 19:50]:
>> On 13 May 2015 at 16:48, Johan Hovold <johan at kernel.org> wrote:
>> > By the way, perhaps you should add a comment in the tps node explaining
>> > why ti,pmic-shutdown-controller must not be disabled (on what revisions
>> > of hardware) as well?
>>
>> There are actually three separate obstacles for RTC-only sleep (only
>> one of which actually BeagleBone-specific), so the "why" is a bit of a
>> long story. The end result can however be stated briefly:
>>
>> Only BBBs prior to rev A6 properly support RTC-only mode. The rare BBW
>> with a 2.x processor also supports it, provided nothing is connected
>> to its expansion ports that leaks significant current from 3v3exp to
>> processor I/Os while in reset.
>>
>> Any BeagleBone with an AM335x 2.x can be patched to properly support
>> RTC-only mode. (Though reconnecting vdds to LDO3 is really no fun, and
>> not a patch I could have done myself.)
>>
>>
>>
>> For reference, the details on the three problems:
>>
>>
>> 1. Most BeagleBone Whites use AM335x 1.0 which freezes the RTC when
>> vdd_core is gone or power-on reset is asserted, rendering RTC-only
>> sleep mostly useless.  The BeagleBone Black, and a few BBWs, use
>> AM335x 2.x which fixed this.
>>
>> Of all problems this is the most benign one since it merely results in
>> loss of functionality. The other two risk hardware damage.
>
> If some of these depend on the SoC revision and cannot be detected
> based on the RTC driver revision register, that information should
> be be passed to the RTC driver in platform data. This can be done
> with the rev == AM35XX_REV_ES* comparison that can be added to the
> mach-omap2/pdata-quirks.c for am335x to initialize platform_data
> for the RTC driver.
>
>> 2. TI issued a notice that the AM335x "vdds" supply must be among the
>> first to come up, and changed the official connection diagram for the
>> TPS65217C/D (used for boards with DDR3/3L memory) by moving vdds from
>> LDO3 to LDO1, and the BBB implemented this change in rev A6A.
>>
>> This change is incompatible with RTC-only sleep (a fact mentioned by
>> TI), so this rules out RTC-only sleep for any AM335x board with
>> DDR3/3L memory unless it predates (or ignored) TI's advice or uses a
>> different power supply scheme altogether. Boards with DDR2 memory
>> (TPS65217A/B) such as the BBW are not affected.
>>
>> Curiously, based on preliminary measurements, I have not observed any
>> leakage to vdds when it is powered up "late" (LDO3). In contrast,
>> during shutdown vdds begins to leak heavily (far in excess of rated
>> max current) to other rails once they drop low enough, and moving vdds
>> to LDO1 only makes this situation persist even longer (and
>> indefinitely in RTC-only sleep). I've asked TI support for some
>> clarification on this, but not yet received any response.
>
> This information should be passed based on the board revision in
> platform_data to the RTC driver assuming we can detect the board
> revision during the early boot.. Otherwise we should have revision
> specific dts files.
>
>> 3. All BeagleBones have a mismanaged external regulator for the
>> "3v3exp" (BBW) / "3v3b" (BBB) rail, which remains enabled longer than
>> the AM335x's 3.3V supplies ("3v3a").
>>
>> On the BBW, external hardware may end up sourcing current into
>> processor I/Os, which feeds the 3v3a through protection diodes. Since
>> the 3v3a is used as enable-signal for the regulator, if non-negligible
>> current is leaked via this path, 3v3a remains "logic high" and the
>> situation will persist indefinitely. If the currents remain modest
>> this won't necessarily violate any absolute maximum ratings, but I'd
>> still worry about the processor's long-term health.
>>
>> On BBB rev A6 and later the situation is the same, but without
>> dependency on external hardware: the current from on-board pull-up
>> resistors already suffices (3v3a remains at ~1.4V), and since rev A6A
>> the leakage from vdds would also get the job done. Worse still, since
>> the buffer for the console port is powered by the 3v3b, having a
>> serial cable attached causes a dangerous amount of current to be
>> driven into UART0_RXD (~45 mA) and I captured an event which I fear
>> may have been a brief latch-up.
>>
>> On BBBs prior to rev A6 the regulator is enabled by LDO2 (whose only
>> other use is the power led), which prevents it from staying on
>> indefinitely. It is however enabled 1ms before and disabled 1ms after
>> the processor's 3.3V supplies, so external hardware must still avoid
>> driving much current into I/O pins during those windows, e.g. by
>> keeping drivers disabled while reset is asserted. As mentioned above,
>> the console port buffer violates this rule.
>>
>> Technically this problem is also present when performing a full
>> shutdown ("OFF-mode"), but then the main supply is cut at the start of
>> the power-down sequence, which proceeds while running off rapidly
>> draining capacitors. Heavy current draw from 3v3b will drain them even
>> faster, thus making the issue self-limiting. (This however does not
>> apply when running on battery power, in which case even a full
>> shutdown is hazardous on an unpatched BBB rev A6 or later.)
>
> Sounds like this should be configured based on the board revision
> too.

All the rev information is in the board's eeprom:

hexdump -e '8/1 "%c"' /sys/bus/i2c/devices/0-0050/eeprom -s 12 -n 4

Rev A5B
0A5B

Rev C
000C

Just another default qwerk to add to Pantelis' bone_capemgr. ;)

Regards,

-- 
Robert Nelson
https://rcn-ee.com/