Hi, jackp@xxxxxxxxxxxxxx writes: > On 2019-10-23 00:49, Felipe Balbi wrote: >> Hi, >> >> Jack Pham <jackp@xxxxxxxxxxxxxx> writes: >>> USB 3.x SuperSpeed peripherals can draw up to 900mA of VBUS power >>> when in configured state. However, if a configuration wanting to >>> take advantage of this is added with MaxPower greater than 500 >>> (currently possible if using a ConfigFS gadget) the composite >>> driver fails to accommodate this for a couple reasons: >>> >>> - usb_gadget_vbus_draw() when called from set_config() and >>> composite_resume() will be passed the MaxPower value without >>> regard for the current connection speed, resulting in a >>> violation for USB 2.0 since the max is 500mA. >>> >>> - the bMaxPower of the configuration descriptor would be >>> incorrectly encoded, again if the connection speed is only >>> at USB 2.0 or below, likely wrapping around UINT8_MAX since >>> the 2mA multiplier corresponds to a maximum of 610mA. >>> >>> Fix these by adding checks against the current gadget->speed >>> when the c->MaxPower value is used and appropriately limit >>> based on whether it is currently at a low-/full-/high- or super- >>> speed connection. >>> >>> Incidentally, 900 is not divisble by 8, so even for super-speed >>> the bMaxPower neds to be capped at 896mA, otherwise due to the >> ^^^^ >> needs >> >> Why do you need to cap it? DIV_ROUND_UP() should still work. > > The round up causes 900 on the input side to be greater than 900 when > doing the > reverse, i.e. multiplication by 8. > > Alternatively we could just do a normal integer division here > (effectively > round down). (...) >> DIV_ROUND_UP(896, 8) = 112 >> DIV_ROUND_UP(900, 8) = 113 >> >> Why value do you want here? ^^^ I mean which, sorry > Right, but now on the host it will do the reverse calculation, i.e. > 113*8 == 904mA. For a root port this would be greater than it's > maximum power budget of 900mA and would result in not selecting the > config. That's a very good explanation of the problem, thank you. It seems like a round down would be safer here in all cases. -- balbi
Attachment:
signature.asc
Description: PGP signature
