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

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Message-ID: <1b1587e8-5c38-4138-a27a-1de71ff07ce3@oss.qualcomm.com>
Date: Tue, 21 Jan 2025 09:50:08 +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 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.

Regards,
Prashanth K