Re: [PATCH] usb: gadget: Increase the limit of USB_GADGET_VBUS_DRAW to 900mA
From: Prashanth K
Date: Tue Jan 21 2025 - 11:49:23 EST
On 21-01-25 08:36 pm, Alan Stern wrote:
> On Tue, Jan 21, 2025 at 09:50:08AM +0530, Prashanth K wrote:
>>
>>
>> On 20-01-25 08:17 pm, Alan Stern wrote:
>>> On Mon, Jan 20, 2025 at 04:47:02PM +0530, Prashanth K wrote:
>>>> Currently CONFIG_USB_GADGET_VBUS_DRAW limits the maximum current
>>>> drawn from Vbus to be up to 500mA. However USB gadget operating
>>>> in SuperSpeed or higher can draw up to 900mA. Also, MaxPower in
>>>> ConfigFS takes its default value from this config. Hence increase
>>>> the allowed range of CONFIG_USB_GADGET_VBUS_DRAW to 900mA.
>>>
>>> Is this the sort of thing that really needs to be a Kconfig option? Why
>>> not make the decision at runtime, based on the needs of the gadget or
>>> function drivers and the connection speed?
>>>
>>> Alan Stern
>>>
>>
>> Right, set_config() in composite.c does this in runtime based on the
>> values of MaxPower (from configFS), VBUS_DRAW defconfig and speed.
>> If we don't set MaxPower from configFS, this config helps to set it
>> during compile time. In fact MaxPower in configFS takes its default
>> value from CONFIG_USB_GADGET_VBUS_DRAW . Sent this patch because Kconfig
>> has this limitation where it's only allowing values upto 500mA.
>
> Why does MaxPower need to be set at compile time? Why not set it at
> runtime instead?
>
> If MaxPower gets set at runtime then it can take its default value to be
> 500 mA or 900 mA depending on the connection speed. There will be no
> need for CONFIG_USB_GAGDGET_VBUS_DRAW.
>
Yes, agreed. Can we mark CONFIG_USB_GAGDGET_VBUS_DRAW as legacy and
maybe also avoid configfs/composite from using it?
> You are trying to improve a compile-time limitation on a quantity which
> isn't known until runtime. The whole idea of doing it that way is
> fundamentally wrong. You shouldn't try to improve it a little, you
> should fix the basic mistake by doing everything at runtime.
>
> Alan Stern
>
Regards,
Prashanth K