linux-kernel - Re: [PATCH] usb: gadget: Increase the limit of USB_GADGET_VBUS

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Message-ID: <158c9087-8252-432d-92a7-dad69add1133@oss.qualcomm.com>
Date: Thu, 20 Feb 2025 09:47:58 +0530
From: Prashanth K <prashanth.k@....qualcomm.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Simona Vetter <simona.vetter@...ll.ch>, Takashi Iwai <tiwai@...e.de>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: gadget: Increase the limit of USB_GADGET_VBUS_DRAW
 to 900mA



On 22-01-25 12:56 am, Alan Stern wrote:
> On Tue, Jan 21, 2025 at 10:19:08PM +0530, Prashanth K wrote:
>>
>>
>> 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?
> 
> Indeed, the whole idea is to avoid using CONFIG_USB_GADGET_VBUS_DRAW in 
> configfs and composite.
> 
> If nothing will still be using it, just remove it entirely.  No need to 
> mark it as legacy.
> 
> Alan Stern

Hi Alan, small doubt, I see that gadget/legacy/webcam.c is a super-speed
gadget which uses CONFIG_USB_GADGET_VBUS_DRAW. I'm quite not really sure
if anyone uses it now, but if someone uses it, then wouldn't my patch be
applicable there?

Noticed this while preparing a patch to remove dependency of VBUS_DRAW
config from configfs/composite layer. Its ready, will send that after
some testing.

Regards,
Prashanth K