Re: [PATCH v2 5/5] power: supply: axp20x_usb_power: set input current limit in probe

[Aren]

Hi Ondřej,

On Wed, Mar 20, 2024 at 01:12:31AM +0100, Ondřej Jirman wrote:
> Hi Aren,
> 
> On Thu, Mar 14, 2024 at 06:39:52PM -0400, Aren wrote:
> > > Also in Pinephone case, you'll not really have a case where the battery has
> > > < 2V not loaded. That's not going to happen. PMIC will shut off at 3V battery
> > > voltage when loaded. It will not discharge further, and after shutoff battery
> > > voltage will jump to 3.4V or so, and it will not drop below 2V after that, ever.
> > > So the battery will pretty much always be detected as long as it's present.
> > 
> > The most likely case I can think of is if someone intentionally tries to
> > boot the device without the battery. I suspect it's also possible for a
> > battery to degrade to the point where it won't hold a charge.
> 
> Yes, that's my usecase that I'd like to preserve. Pinephone has removable
> battery and using it without battery is quite reasonable. It works fine
> currently for me and this patch will break this if there's no opt out. And
> there's no opt out other than patching and re-building the kernel.
> 
> > > What actual problem have you seen that this patch is trying to solve?
> > 
> > The problem, in theory, is that the pmic ignores the USB BC
> > specification and sets the current limit to 3A instead of 500mA. In
> > practice (as long as the power supply is implemented properly) if this
> > is too much power, it should just cause the power supply to shut off.
> > I'm not sure how likely / what the risks of a power supply cutting
> > corners are.
> 
> Pinephone with no battery takes between 0.5-1A from VBUS. Even under full
> load, it's not enough to damage even a USB 3.0 SDP port. It's only a
> slight problem for unprotected USB 2.0 SDP ports. On protected 2.0 ports
> or ports with overdesigned output power, it will either shut down due
> to brownout, or just work.
> 
> It's not enough to overload any actual USB charger.
> 
> In any case, people wanting to run Pinephone without a battery probably
> will not do so from a USB 2.0 computer port. Maybe for FEL USB mode, for
> development or flashing, but at that stage the power consumption is still
> very low, well below 2.5W.
> 
> > I find it surprising that the hardware/driver takes a lot of care to
> > figure out what the proper current is and stick to that, except when
> > there isn't a battery.
> 
> It seems to have apparent purpose documented in the datasheet:
> 
>   https://megous.com/dl/tmp/78d4c0771fc6d2c8.png
> 
> "If Battery not present, and this bit is 0,the VBUS current limit set to 3A,
> for the F/W update in factory"
> 
> You can also get rid of the issue by writing 1 to:
> 
>   "REG 2DH: BC Module VBUS Control and Status Register" bit 6
> 
> in the bootloader very early on before enabling BC. That should fix the issue,
> too in a more proper way than forcing 500mA limit halfway during boot, when
> BC1.2 detection might have detected something higher earlier on and the boot
> success depends on the higher value.

I agree this is a better way of handling this, I made an attempt
previously to get USB BC working without a battery connected, but I was
has having problems with USB BC hanging. Based on what you describe
below, I now suspect that had to do with leaving dead battery detection
enabled (0x2E bit 6). I'll take a look through the code again to see if
I can get it working.

Thanks
 - Aren

> > battery. The datasheet says that register 0x2D bit 6 is used to indicate
> > first power on status. According to it, if that bit is 0 and the battery
> > is not detected, it will set the input current limit to 3A, however
> > setting that bit to 1 doesn't to prevent the pmic from setting the
> > current limit to 3A.
> 
> Actually it does (I made a quick test with Pinephone with no battery being
> plugged to the PC's USB port and executing a test program over FEL that talks to
> the PMIC):
> 
> PMIC registers: (initial values post-powerup with no battery)
> 
> 2c: 0    - BC disabled by default, something has to enable it
> 2d: b0   - bit 6 not set (*not* first boot bit)
> 2e: 40
> 2f: 0
> 30: 1
> 31: 3
> 32: 43
> 33: c5
> 34: 45
> 35: 68   - initially 3A limit
> 36: 59
> 37: 0
> 38: a5
> 39: 1f
> 3a: 80
> 
> changed values
> 
> 2c: 5    - test program enables BC
> 2e: 0    - disable DB detection (otherwise with no battery DB detection will
>            prolong BC detection result by 45minutes or whatever is the timeout)
> 	   see DBP_Timeout_CTL(DBP Hardware Timeout Control)
> 2f: 10
> 30: 2a
> 35: 38   - test program sets 1.5A limit
> 36: 8
> 
> ... about 400 ms later
> 
> 2c: 1    - BC complete
> 2f: 30   - BC result = SDP (matches reality)
> 35: 68   - 3A VBUS limit set by PMIC itself
> 
> 
> Another run with 2d.6 bit set:
> 
> (initial values omitted, same as above)
> 
> 
> changed values
> 
> 2c: 5   - enable BC
> 2d: f0  - bit 6 set (*not* first boot)
> 2e: 0
> 2f: 10
> 30: 2a
> 35: 38   - test program sets 1.5A limit
> 36: 8
> 
> ... after about 400 ms
> 
> 2c: 1   - BC complete
> 2f: 30  - BC result = SDP
> 35: 18  - 500mA VBUS limit set by PMIC itself
> 
> 
> So the detection works with no battery inserted. PMIC's BC correcly detects
> regular USB 2.0 data port and configures a correct limit in about 400ms
> after cable plug in. So I don't see a problem with the HW, that you're
> describing in the commit message.
> 
> The proper way to handle this issue is to fix whichever component is configuring
> the BC detection initially (it's disabled by default, apparently), to properly
> set the first boot bit before enabling BC. IMO, that place should be the
> platform firmware. Then the detection will work from the get go and proper limit
> will be always set correctly and will match the USB charger, and there will be
> no need for any kernel hacks.
> 
> Pretty much what platform FW should do is to:
> 
> - set "not first boot bit" in 0x2d register
> - check if battery is present
>   - if not clear 0x2e register
>   - otherwise configure DBD in 0x2e
> - configure DCP/CDP current limit to 1.5A or 2A (1.5A maybe safer)
> - configure VBUS Vhold to 4.5V (Pinephone needs this for powered USB dock to
>   work with in general with arbitrary chargers, and it will overload weaker
>   USB PSU's less)
> - configure BC detection and start it
> 
> My usecase of using Pinephone without battery will still work, too, and will
> not be broken by this patch.
> 
> > The point of this patch (after a revision) should be to make it explicit
> > when and why this driver ignores the USB BC specification. And to reduce
> > the cases where it does, if possible.
> 
> The kernel has no business forcing the limit to some fixed low value that
> has no relationship to the last BC detection result and breaks boot in
> the exact scenario this patch is targetting (no battery -> too high current
> limit).
> 
> This doesn't make any sense to me.
> 
> kind regards,
> 	o.
> 
> > With the goal of making it explicit what cases ignore the spec, I would
> > prefer to have an opt-out mechanism. I compiled what I believe to be a
> > full list of devices that use this driver with usb bc enabled (detailed
> > notes below), and there's only a handful of them. It shouldn't be too
> > difficult to out-out the boards that need it.
> > 
> > > 
> > > Thank you and kind regards,
> > > 	o.
> > 
> > Sorry it took me a while to respond, I haven't had much time to work on
> > this in the past few weeks.
> > 
> > Regards
> >  - Aren
> > 
> > p.s. the notes on what devices use this functionality:
> > 
> > These devices include the axp803 or axp81x dtsi:
> > $ rg -l 'include "axp(803|81x).dtsi"'
> >  - sun50i-a100-allwinner-perf1.dts
> >  - sun50i-a64-amarula-relic.dts
> >  - sun50i-a64-bananapi-m64.dts
> >  - sun50i-a64-nanopi-a64.dts
> >  - sun50i-a64-olinuxino.dts
> >  - sun50i-a64-orangepi-win.dts
> >  - sun50i-a64-pine64.dts
> >  - sun50i-a64-pinebook.dts
> >  - sun50i-a64-pinephone.dtsi
> >  - sun50i-a64-pinetab.dts
> >  - sun50i-a64-sopine.dtsi
> >  - sun50i-a64-teres-i.dts
> >  - sun8i-a83t-allwinner-h8homlet-v2.dts
> >  - sun8i-a83t-bananapi-m3.dts
> >  - sun8i-a83t-cubietruck-plus.dts
> >  - sun8i-a83t-tbs-a711.dts
> > 
> > Out of those only these enable usb_power_supply:
> > $ rg -l 'include "axp(803|81x).dtsi"' | xargs rg -l 'usb_power_supply'
> >  - sun50i-a64-bananapi-m64.dts
> >  - sun50i-a64-pinetab.dts
> >  - sun50i-a64-pinephone.dtsi
> >  - sun8i-a83t-tbs-a711.dts
> >  - sun8i-a83t-cubietruck-plus.dts
> >  - sun8i-a83t-bananapi-m3.dts
> > 
> > sun50i-a64-bananapi-m64.dts: The barrel jack is connected to acin, so
> > will be unaffected. Banannapi docs say it's not possible to power over
> > usb, but schematic suggests it should work. Probably needs to opt-out of
> > the lower current limit.
> > 
> > sun50i-a64-pinetab.dts: unclear if charging is supported via usb, vbus
> > is connected through a component listed as "NC/0R". Regardless device
> > has barrel jack and battery for power, shouldn't need to run exclusively
> > from usb.
> > 
> > sun50i-a64-pinephone.dtsi: is typically booted with a battery connected,
> > shouldn't need to run exclusively from usb.
> > 
> > sun8i-a83t-tbs-a711.dts: has an internal battery, shouldn't need to run
> > exclusively from usb.
> > 
> > sun8i-a83t-cubietruck-plus.dts and sun8i-a83t-bananapi-m3.dts: Both
> > appear to support being powered over usb and a barrel jack. These will
> > need to opt-out to be able to run from usb.