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

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Message-ID: <5252b43d-a7ee-42b5-932e-6c6eadd04543@rock-chips.com>
Date: Mon, 25 Aug 2025 11:35:10 +0800
From: William Wu <william.wu@...k-chips.com>
To: Alan Stern <stern@...land.harvard.edu>
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 8/22/2025 9:56 PM, Alan Stern wrote:
> 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
Good idea，I checked the UDC drivers (drivers/usb/gadget/udc and 
dwc2/dwc3), they test if zlp is actually needed if req->zero to 1.>
>>   	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
> 
> 
>