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

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Date:	Thu, 25 Nov 2010 13:08:42 +0100
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Tomoya MORINAGA <tomoya-linux@....okisemi.com>
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 11/25/2010 01:03 PM, Tomoya MORINAGA wrote:
> 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 ?

Why do you Read-Modify-Write for TX? Naively speaking you just need to
push your Data into a Mail/IF/Whatever and push the send button.

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
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