netdev - Re: [PATCH v5] net: ethernet: add driver for Aurora VLSI NB8800 Ethernet controller

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Date:	Wed, 11 Nov 2015 14:26:50 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	mans@...sr.com
Cc:	romieu@...zoreil.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, slash.tmp@...e.fr
Subject: Re: [PATCH v5] net: ethernet: add driver for Aurora VLSI NB8800
 Ethernet controller

From: Måns Rullgård <mans@...sr.com>
Date: Wed, 11 Nov 2015 19:25:46 +0000

> David Miller <davem@...emloft.net> writes:
> 
>> From: Måns Rullgård <mans@...sr.com>
>> Date: Wed, 11 Nov 2015 19:17:07 +0000
>>
>>> David Miller <davem@...emloft.net> writes:
>>> 
>>>> From: Måns Rullgård <mans@...sr.com>
>>>> Date: Wed, 11 Nov 2015 19:09:19 +0000
>>>>
>>>>> David Miller <davem@...emloft.net> writes:
>>>>> 
>>>>>> From: Måns Rullgård <mans@...sr.com>
>>>>>> Date: Wed, 11 Nov 2015 18:25:05 +0000
>>>>>>
>>>>>>> If the TX DMA channel is idle when start_xmit is called, it can be
>>>>>>> started immediately.  Checking the DMA status and starting it if
>>>>>>> idle has to be done atomically somehow.
>>>>>>
>>>>>> ->ndo_start_xmit() is guaranteed to be invoked atomically, protected
>>>>>> by the TX queue spinlock.
>>>>> 
>>>>> Yes, but the DMA needs to be restarted from some other context if it was
>>>>> busy when start_xmit checked.
>>>>
>>>> Then you can probably use the TXQ lock in the interrupt handler just for
>>>> that.
>>> 
>>> That seems a bit heavy-handed when the critical section for this is only
>>> a tiny part of the start_xmit function.
>>
>> Then what synchornization primitive other than spin locks are you going
>> to use for this?
>>
>> My point is that there is a spinlock the core code is _already_ taking,
>> unconditionally, when ->ndo_start_xmit() executes.  And you can therefore
>> take advantage of that rather than using another lock of your own.
> 
> I get that.  But that remains locked for the duration of ndo_start_xmit()
> whereas the part that needs to be synchronised with the DMA completion
> IRQ handler is tiny.  Having the IRQ handler spin for the duration of
> ndo_start_xmit() seemed silly to me.

I don't think it's silly at all.

And unless you can measure it making a difference, don't knock the idea.
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