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

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Message-ID: <e823a961-a0a1-46c3-84a9-7da3cd718f4c@rowland.harvard.edu>
Date: Tue, 21 Jan 2025 14:26:11 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: Prashanth K <prashanth.k@....qualcomm.com>
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 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