netdev - Re: [PATCH v6 0/2] Ethernet drivers for WIZnet chips

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Message-ID: <4F7BE30B.9020603@permonline.ru>
Date:	Wed, 04 Apr 2012 11:58:35 +0600
From:	Mike Sinkovsky <msink@...monline.ru>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 0/2] Ethernet drivers for WIZnet chips

03.04.2012 18:29, Eric Dumazet написал:
> On Tue, 2012-04-03 at 18:15 +0600, Mike Sinkovsky wrote:
>> 03.04.2012 17:39, Eric Dumazet написал:
>>> On Tue, 2012-04-03 at 16:58 +0600, Mike Sinkovsky wrote:
>>>> Based on original driver from chip manufacturer, but nearly full rewite.
>>>> Tested and used in production with Blackfin BF531 embedded processor.
>>>>
>>>> Signed-off-by: Mike Sinkovsky<msink@...monline.ru>
>>>> ---
>>>> v6:
>>>>    - remove (S0_TX_FSR<   skb->len) check in TX handler, it doesn't work
>>>>      anyway. Chip can transmit only one frame with MTU 1500 at a time,
>>>>      and tx buffer size is bigger.
>>>
>>> So what happens if XXX frames are given to start_xmit() in a flood ?
>>>
>>> You removed any flow control, how can this work ?
>>>
>>> Device has an infinite queue ?
>>
>> As I understand from datasheet, device doesn't have tx queue at all.
>> It have tx buffer, processor must save transmitted frame to it, and then
>> save SEND command to command register. When transmission completed,
>> SENDOK bit in interrupt register will be set to '1', and interrupt
>> handler will be called.
>>
>> So, according to datasheet, driver must stop queue in start_xmit()
>> routine, and wake in interrupt handler of SENDOK bit.
>> I tried this, and it basically works, but SOMETIMES, very rare, I see
>> tx_timeout().
>>
>> And without any flow control - driver works perfectly.
>> Weird, don't know why.
>
> Really this should be fixed, since your driver makes qdisc flow control
> impossible (unless adding a rate limiter like HTB/CBQ)
>
> You probably had a race on your xmit routine, and interrupt routine.
>
> You need proper synchronization : A spinlock to make this easy, or a
> smart barrier game.

I can at least make it configurable from Kconfig with default 'y', and 
disable for our boards.
Would it be acceptable?

About races - I can't see any, in this very simple procedure.
But maybe I missed something..


-- 
Mike

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