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

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Date:	Wed, 11 Nov 2015 19:25:46 +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 19:17:07 +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.

-- 
Måns Rullgård
mans@...sr.com
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