linux-kernel - Re: [PATCH] usb: gadget: f_hid: Fix zero length packet transfer

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Message-ID: <c09f21ee-be83-4880-841b-70e1e5c036f0@rowland.harvard.edu>
Date: Fri, 22 Aug 2025 09:56:49 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: William Wu <william.wu@...k-chips.com>
Cc: gregkh@...uxfoundation.org, Chris.Wulff@...mp.com,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	frank.wang@...k-chips.com, jianwei.zheng@...k-chips.com,
	yue.long@...k-chips.com
Subject: Re: [PATCH] usb: gadget: f_hid: Fix zero length packet transfer

On Fri, Aug 22, 2025 at 10:01:58AM +0800, William Wu wrote:
> If the hid transfer with size divisible to EPs max packet
> size, it needs to set the req->zero to true, then the usb
> controller can transfer a zero length packet at the end
> according to the USB 2.0 spec.
> 
> Signed-off-by: William Wu <william.wu@...k-chips.com>
> ---
>  drivers/usb/gadget/function/f_hid.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_hid.c b/drivers/usb/gadget/function/f_hid.c
> index 8e1d1e8..8021af3 100644
> --- a/drivers/usb/gadget/function/f_hid.c
> +++ b/drivers/usb/gadget/function/f_hid.c
> @@ -511,7 +511,7 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
>  	}
>  
>  	req->status   = 0;
> -	req->zero     = 0;
> +	req->zero     = ((count % hidg->in_ep->maxpacket) == 0);

Here and below, there is no need to check whether count is divisible by 
the maxpacket length.  The UDC driver does this for you automatically.  
(See the kerneldoc for struct usb_request.)  Simply set req->zero to 1.

Alan Stern

>  	req->length   = count;
>  	req->complete = f_hidg_req_complete;
>  	req->context  = hidg;
> @@ -967,7 +967,7 @@ static int hidg_setup(struct usb_function *f,
>  	return -EOPNOTSUPP;
>  
>  respond:
> -	req->zero = 0;
> +	req->zero = ((length % cdev->gadget->ep0->maxpacket) == 0);
>  	req->length = length;
>  	status = usb_ep_queue(cdev->gadget->ep0, req, GFP_ATOMIC);
>  	if (status < 0)
> -- 
> 2.0.0