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

[David Miller]

From: Måns Rullgård <mans@xxxxxxxxx>
Date: Wed, 11 Nov 2015 19:17:07 +0000

> David Miller <davem@xxxxxxxxxxxxx> writes:
> 
>> From: Måns Rullgård <mans@xxxxxxxxx>
>> Date: Wed, 11 Nov 2015 19:09:19 +0000
>>
>>> David Miller <davem@xxxxxxxxxxxxx> writes:
>>> 
>>>> From: Måns Rullgård <mans@xxxxxxxxx>
>>>> 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.
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