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

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Date:	Wed, 11 Nov 2015 19:35:05 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	David Miller <davem@...emloft.net>
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

David Miller <davem@...emloft.net> writes:

> 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.

I'm sure I read somewhere that the time spent spinning on a lock should
be kept as small as possible.

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

I tried using netif_tx_lock() in the IRQ handler instead, and it locked
up solid.  Clearly that was the wrong thing to do.

-- 
Måns Rullgård
mans@...sr.com
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