On Mon, Mar 25, 2019 at 4:58 PM Hans de Goede <hdegoede@xxxxxxxxxx> wrote: > > Hi, > > On 25-03-19 03:45, Chen-Yu Tsai wrote: > > On Thu, Mar 21, 2019 at 5:30 PM Maxime Ripard <maxime.ripard@xxxxxxxxxxx> wrote: > >> > >> Hi, > >> > >> The rest of the series is > >> Acked-by: Maxime Ripard <maxime.ripard@xxxxxxxxxxx> > >> > >> On Thu, Mar 21, 2019 at 04:48:44PM +0800, Chen-Yu Tsai wrote: > >>> From: Chen-Yu Tsai <wens@xxxxxxxx> > >>> > >>> The AXP PMICs allow the user to disable current limiting on the VBUS > >>> input. While read-out of this setting was already supported by the > >>> driver, it did not allow the user to configure the PMIC to disable > >>> current limiting. > >>> > >>> Add support for this. > >>> > >>> Signed-off-by: Chen-Yu Tsai <wens@xxxxxxxx> > >> > >> Do we really want to do that though? That could have some pretty bad > >> consequences. > > > > If I understand the manual correctly, the PMIC has two mode of operation > > with regards to VBUS. Normal operation means the PMIC will try to limit > > the current draw to maintain VBUS above the set V_hold (defaults to 4.4V). > > This is in addition to the current limit set in this patch. > > > > The other mode of operation is bypass, where it ignores the voltage limit. > > Not sure if it also ignores the current limit, but probably not. In any > > case we don't support this mode in the driver. > > > > So I can think of a few cases where this might be bad: > > > > 1. High current draw results in excessive voltage drop and heating over > > line / traces due to insufficient conductor area. This should be covered > > by the voltage holding mechanism. > > > > 2. Over taxing the external power supply. This should also result in some > > voltage drop for simple power bricks. Advanced ones would either have > > current limiting or over-current protection. > > > > What bad consequences are you thinking of? > > Lets say you use a typical 5v / 2A charger-plug, lets also say that at full > load this brick has an efficiency of 90%. At full load it delivers 10W of > power, while consuming 11.1W dissipating 1.1W of losses as heat. > > Now lets say we disable current-limiting and rely only on the V_hold > mechanism, lets say that we end up with 4.5 volts at 2.4 amps because of > this and since we are now operating in overload conditions the > efficiency has fallen to 80% (approx. 4.5/5.0 * 90%) so now at full load > it delivers 10.8W of power, while consuming 13.5W dissipating 2.7W of > losses as heat. Chances are the the small plastic enclosure of your > typical charger-plug cannot dissipate this much and will start warming > up, until it bursts into flames. > > Disabling current limit protection is a very bad idea because you will > end up in an equilibrium between the Vhold from the charger-ic and the > over-current protection from the power-brick where you are over the > design limit of the power-brick. > > I actually like what the TI charger-ics are doing here a lot more then > what the AXP series is doing, with TI charger-ics if you set a current > limit > 500mA and the power-brick's voltages drops too much because of > this (or because of a bad cable) it automatically falls back to 500mA. > Where as at least with the AXP288, it simply starts drawing 1.5A at 4.5V > with a bad cable, but in this case the dissipation at least is happening > inside the cable rather then inside the charger-plug, which typically > already gets quite hot under normal operation conditions. > > Disabling the current limit is basically the same as what bad USB-A > to USB-C cables which have a Rp-3.0 resistor in the C plug do, these tell > the device with the Type-C plug it can safely draw 3A from the A-port the > A plug is plugged into. The web is full of stories about this causing > damage to machines, e.g.: > > "Bohn's Nexus 6P drew too much power from his MacBook Air while using a third-party cord, frying the machine and making the USB Type-C ports work only intermittently." > > From: https://www.laptopmag.com/articles/how-to-find-safe-usb-type-c-cables > > Another good link about the problems caused by these bad Rp resistor > values in Type-C to Type-A cables (which also effectively disable the > current-limit on the device charging on the C-end of the cable): > https://plus.google.com/102617628172847077584/posts/HakwCMmd346 > > Note this second link is going away in 6 days as google is retiring > google+. > > Anyways TL;DR: Disabling the current-limit is a bad idea and really > nothing good can come from this. OK. I'll respin the series without this. ChenYu
