netdev - Re: [PATCH RFC] 2/2 huawei_cdc_ncm: introduce new TX ncm stack

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Date:	Fri, 26 Jun 2015 10:14:02 +0200
From:	Oliver Neukum <oneukum@...e.com>
To:	Enrico Mioso <mrkiko.rs@...il.com>
Cc:	linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC] 2/2 huawei_cdc_ncm: introduce new TX ncm stack

On Thu, 2015-06-25 at 17:19 +0200, Enrico Mioso wrote:
> Hi Oliver! And thank you again.
> 
> I like / recommend the usage of open messaging standards: my preferred XMPP ID
> (JID) is: mrkiko@....si.
> 
> On Thu, 25 Jun 2015, Oliver Neukum wrote:
> 
> > Date: Thu, 25 Jun 2015 15:38:46
> > From: Oliver Neukum <oneukum@...e.de>
> > To: Enrico Mioso <mrkiko.rs@...il.com>
> > Cc: linux-usb@...r.kernel.org, netdev@...r.kernel.org
> > Subject: Re: [PATCH RFC] 2/2 huawei_cdc_ncm: introduce new TX ncm stack
> > 
> > On Thu, 2015-06-25 at 13:44 +0200, Enrico Mioso wrote:
> >
> >> On Thu, 25 Jun 2015, Oliver Neukum wrote:
> >
> >>> Is there any advantage in keeping this in a single function?
> >>>
> >> I did this choice in the light of the fact I think the tx_fixup function will
> >> become more complex than it is now, when aggregating frames.
> >
> > Yes, but that is a reason to split the helpers up not the opposite.
> Right - I understood only now your observation.
> 
> the only reason to write the manager that way was that it shouldn't become very 
> complex - it should simply do things to frames, helping out in building valid 
> NCM packages.
> 
> >
> >> I answer here your other message to make it more convenient to read: my
> >> intention for the tx_fixup function would be to:
> >> - aggregate frames
> >> - send them out when:
> >>  	- a timer expires
> >
> > How would you do that in tx_fixup()? If a timer is required then you
> > need a separate function.
> >
> Sure. I meant: I will adapt it in case needed, and expectin the code to become 
> a little bit more convoluted.

You cannot become any more convoluted even if you try.

> >>  	OR
> >>  	- we have enough data in the aggregate, and cannot add more.
> >
> > Yes.
> >
> > You need a third case:
> > 	- the interface is taken down.
> >
> > But in general the logic for that is already there. So can you explain
> > what additional goals you have?
> regarding the "timer logic" I saw it in cdc_ncm.c, but I should study it in 
> more detail to understand it and implement it here where needed in case.

It is involved. Basically a timer for transmission creates locking
issues. cdc-ncm uses an hrtimer which calls a bottom half which
calls xmit with a NULL skb.

        /* if there is a remaining skb, it gets priority */
        if (skb != NULL) {
                swap(skb, ctx->tx_rem_skb);
                swap(sign, ctx->tx_rem_sign);
        } else {
                ready2send = 1;
        }

The else branch here is the timer triggering.
The actual transmission is done in usbnet if cdc_ncm_fill_tx_frame()
returns an skb.

I doubt you can can come up with anything more convoluted
but it simplifies locking.

> > Well, no, but it supposes a matched commit phase. Can you guarantee
> > that? I was under the oppression that in that phase you want to actually
> > give a frame over to the hardware.
> No. When Italk about committing, I think about "writing things to out skb".
> another reason why i found confortable writing the code this way was to 
> maintain a kind of statefullness in a more "clean" way.
> But I understand this is kind of agruable, and I can if needed break it up as 
> desired.
> 
> Regarding the commit phase - I am not sure I understand your comment, sorry
> about that.
> However, my intention would be to allow the caller to do calls in sequences 
> like:
> - init frame
> - ncm update
> - ncm update
> - ncm update
> ...
> - ncm update
> - ncm commit
> 
> (to work in "huawei" mode)
> 
> OR
> 
> - ncm init frame
> - ncm commit
> - ncm update
> - ncm update
> - ncm update
> - ncm update
> - finalize nth
> 
> (to work in "cdc_ncm.c"-mode)..

Sounds like a plan.

> But to prevent usbnet from submittinx RX'd packets, I should be doing something 
> nasty here. And unfortunately don't understand why.

        // some devices want funky USB-level framing, for
        // win32 driver (usually) and/or hardware quirks
        if (info->tx_fixup) {
                skb = info->tx_fixup (dev, skb, GFP_ATOMIC);
                if (!skb) {
                        /* packet collected; minidriver waiting for more
*/
                        if (info->flags & FLAG_MULTI_PACKET)
                                goto not_drop;

So you just return NULL if you are ready to queue more. But you
need the FLAG_MULTI_PACKET flag.

	Regards
		Oliver


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