linux-kernel - Re: [PATCH v1 2/2] usb: gadget: composite: Draw 100mA current if not configured

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Date:   Wed, 22 Feb 2023 23:33:29 -0800
From:   Jack Pham <quic_jackp@...cinc.com>
To:     Prashanth K <quic_prashk@...cinc.com>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Jakob Koschel <jakobkoschel@...il.com>,
        Jó Ágila Bitsch <jgilab@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Pratham Pratap <quic_ppratap@...cinc.com>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 2/2] usb: gadget: composite: Draw 100mA current if not
 configured

Hi Prashanth,

On Thu, Feb 23, 2023 at 10:48:30AM +0530, Prashanth K wrote:
> Currently we don't change the current value if device isn't in
> configured state. But the battery charging specification says,
> the device can draw upto 100mA of current if its in unconfigured

Here you say spec says "up to" (BTW you have a typo) 100mA...

> state. Hence add a Vbus_draw work in composite_resume to draw
> 100mA if the device isn't configured.

But here and in the patch you are calling the function to draw exactly
100mA.  Consider the possibility that a gadget could be configured to
draw less current than that or not anything at all, we should make sure
to honor that as an absolute maximum.

> Signed-off-by: Prashanth K <quic_prashk@...cinc.com>
> ---
>  drivers/usb/gadget/composite.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
> index fa7dd6c..147d278 100644
> --- a/drivers/usb/gadget/composite.c
> +++ b/drivers/usb/gadget/composite.c
> @@ -2531,6 +2531,8 @@ void composite_resume(struct usb_gadget *gadget)
>  			usb_gadget_clear_selfpowered(gadget);
>  
>  		usb_gadget_vbus_draw(gadget, maxpower);
> +	} else {
> +		usb_gadget_vbus_draw(gadget, 100);

Similar to the configured case, maybe you can perform a min()
calculation against either or both the config->MaxPower or
CONFIG_USB_GADGET_VBUS_DRAW.

Thanks,
Jack