linux-kernel - Re: [PATCH net-next-2.6 v3] can: Topcliff: PCH

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Date:	Thu, 25 Nov 2010 21:03:45 +0900
From:	"Tomoya MORINAGA" <tomoya-linux@....okisemi.com>
To:	"Marc Kleine-Budde" <mkl@...gutronix.de>
Cc:	<andrew.chih.howe.khor@...el.com>,
	<socketcan-core@...ts.berlios.de>,
	"Samuel Ortiz" <sameo@...ux.intel.com>, <margie.foster@...el.com>,
	<netdev@...r.kernel.org>,
	"Christian Pellegrin" <chripell@...e.org>,
	<linux-kernel@...r.kernel.org>, <yong.y.wang@...el.com>,
	<kok.howg.ewe@...el.com>, <joel.clark@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Wolfgang Grandegger" <wg@...ndegger.com>, <qi.wang@...el.com>
Subject: Re: [PATCH net-next-2.6 v3] can: Topcliff: PCH_CAN driver: Add Flow control,

On Wednesday, November 24, 2010 9:34 PM, Marc Kleine-Budde wrote :
>On 11/24/2010 01:09 AM, Tomoya MORINAGA wrote:
>> On Monday, November 22, 2010 5:27 PM, Marc Kleine-Budde wrote:
>>>>>>> Still we have the busy waiting in the TX path. Maybe you can move the
>>>>>>> waiting before accessing the if[1] and remove the busy waiting here.
>>>>>> I can't understand your saying.
>>>>>> For transmitting data, calling pch_can_rw_msg_obj is mandatory.
>>>>> Yes, but the busy wait is not needed. It should be enough to do the
>>>>> busy-waiting _before_ accessing the if[1].
>>>>
>>>> Do you mean we should create other pch_can_rw_msg_obj which doesn't have busy wait ?
>>> ACK, and this non busy waiting is use in the TX path. But you add a busy
>>> wait only function before accessing the if[1] in the TX path.
>> 
>> The "busy waiting" of pch_can_rw_msg_obj is for next processing accesses to Message object.
>> If deleting this busy waiting, next processing can access to Message object, regardless previous transfer doesn't
>> complete yet.
>> Thus, I think, the "busy waiting" is necessary.
>
>Yes, it's necessary, but not where it is done currently.
>Let me outline how I think the TX path should look like:
>
>pch_xmit() {
> take_care_about_flow_control();
> prepare_can_frame_to_be_copied_to_tx_if();
>
> /* most likely we don't have to wait here */
> wait_until_tx_if_is_ready();
>
> copy_can_frame_to_tx_if();
>
> /* trigger tx in hardware */
> send_tx_if_but_dont_do_busywait();
>
> /* tx_if is busy now, but before we access it, we'll check tx_if is ready */
>}

This Tx path also has Read-Modify-Write for MessageRAM access.
Do you mean Tx path shouldn't have Read-Modify-Write ?


---
Thanks,

Tomoya MORINAGA
OKI SEMICONDUCTOR CO., LTD.
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