netdev - Re: [PATCH net-next-2.6 v6 04/20] can: EG20T PCH: Add Tx Flow Control

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Date:	Fri, 3 Dec 2010 09:58:59 +0900
From:	"Tomoya MORINAGA" <tomoya-linux@....okisemi.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	<wg@...ndegger.com>, <w.sang@...gutronix.de>, <chripell@...e.org>,
	<21cnbao@...il.com>, <sameo@...ux.intel.com>,
	<socketcan-core@...ts.berlios.de>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <qi.wang@...el.com>,
	<yong.y.wang@...el.com>, <andrew.chih.howe.khor@...el.com>,
	<joel.clark@...el.com>, <kok.howg.ewe@...el.com>,
	<margie.foster@...el.com>
Subject: Re: [PATCH net-next-2.6 v6 04/20] can: EG20T PCH: Add Tx Flow Control

On Friday, December 03, 2010 6:12 AM, David Miller wrote :

> 
> When implementing functionality like this it is better to use other
> existing well tested network drivers as a guide rather then trying
> to be unique and clever, as you are doing here.

Sure. I will do so.

> 
> Second of all, checking the state of the device to determine if a
> stop queue should be performed has two problems:
> 
> 1) The test uses a magic constant mask, which is undocumented.

Do you above "magic constant" mean "0xfc00" ?
If so, this constant is replaced to MACRO in 08/20 patch. 

> 
> 2) It causes the race you have on the wake queue side
> 
> Use pure software state to guide your actions, and let the hardware
> interrupt trigger the wake queue.
> 
> Also, you don't implement this as a true ring buffer, you only
> consider to stop the queue when you hit the last TX object entry.  But
> all the previous slots could be available.

We can't use ring buffer.
Because ring buffer causes issue of tx packet out-of-order.

I show EG20T CAN HW spec below
According to priority, EG20T CAN HW sends packet from message object.
The priority is Message object number.
Lower number is high-priority, Higher is low priority

Thus, I think we can't use ring buffer.

Still, do you think I can use ring buffer ?
------
Thanks,

Tomoya MORINAGA
OKI SEMICONDUCTOR CO., LTD.
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