netdev - Re: [PATCH net] net: cdc_ncm: workaround for missing CDC Union

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Date:	Mon, 21 Jan 2013 16:28:48 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	Alexey Orishko <alexey.orishko@...il.com>, netdev@...r.kernel.org,
	linux-usb@...r.kernel.org, Greg Suarez <gsuarez@...thmicro.com>,
	Alexey Orishko <alexey.orishko@...ricsson.com>
Subject: Re: [PATCH net] net: cdc_ncm: workaround for missing CDC Union

Oliver Neukum <oneukum@...e.de> writes:
> On Monday 21 January 2013 15:47:13 Bjørn Mork wrote:
>> But I wonder if this isn't really a generic problem in usbnet.  The
>> FLAG_MULTI_PACKET test here seems completely bogus:
>> 
>>         if (length % dev->maxpacket == 0) {
>>                 if (!(info->flags & FLAG_SEND_ZLP)) {
>>                         if (!(info->flags & FLAG_MULTI_PACKET)) {
>>                                 urb->transfer_buffer_length++;
>>                                 if (skb_tailroom(skb)) {
>>                                         skb->data[skb->len] = 0;
>>                                         __skb_put(skb, 1);
>>                                 }
>>                         }
>>                 } else
>>                         urb->transfer_flags |= URB_ZERO_PACKET;
>>         }
>> 
>> Either the FLAG_MULTI_PACKET minidriver will have already padded the
>> buffer so that we do not hit (length % dev->maxpacket == 0), or we
>> should choose one of the alternatives: ZLP or padding.
>
> But we cannot simply call __skb_put for a complicated data frame.

Agreed.  But I believe the condition should be 

 if (!(info->flags & FLAG_SEND_ZLP) && !(info->flags & FLAG_MULTI_PACKET)) {
     ..
 } else {
     urb->transfer_flags |= URB_ZERO_PACKET;
 }

to ensure that we send the ZLP in this case.

> Besides you may want the current behavior.

Why? Does it ever make sense to prevent both the short packet and the
ZLP?


Bjørn
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