netdev - Re: [RFC PATCH v2 0/2] nb8800 suspend/resume support

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Message-ID: <yw1xo9rygdyu.fsf@mansr.com>
Date:   Wed, 02 Aug 2017 16:56:57 +0100
From:   Måns Rullgård <mans@...sr.com>
To:     Mason <slash.tmp@...e.fr>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH v2 0/2] nb8800 suspend/resume support

Mason <slash.tmp@...e.fr> writes:

> On 02/08/2017 17:36, Måns Rullgård wrote:
>
>> Mason wrote:
>> 
>>> Looking at the tango-specific integration, I note this nugget:
>>>
>>> 1.5.4 Stopping & Starting the DMA
>>>
>>> This feature has been added to allow the software to stop and start
>>> the DMA without any issues.
>>>
>>> Procedure:
>>> 1- STOP:
>>> 2- Stop RX core;
>>> 3- Set OWN bit of all descriptor of the chain to 1;
>>> 4- Stop DMA by writing dma_stop bit to 1 in  RX_DMA_Stop register
>>> 5- Wait around 100 clock cycles.
>>>
>>> The pending packets are held until the system will re-start.
>>>
>>> RE-START:
>>> 1- Clear dma_stop bit (note that if at the time of stopping the DMA,
>>> the next packet in the FIFO was an UDP packet, when clearing dma_stop,
>>> this packet will directly start being written in the DRAM since UDP
>>> packets are not controlled by the descriptor mechanism);
>>> 2- Program a new chain of descriptor;
>>> 3- Re-enable DMA (rx_ctrl register)
>>>
>>> rx_dma_stop:
>>> Software control to stop the Rx DMA.
>>> A write to this bit with "1" will gracefully stop the Rx DMA by after
>>> transferring the current packet. If more packets are pending they will
>>> be held until the software clears this bit.
>>>
>>> Hmmm, what do you think? This looks promising...
>> 
>> This is only available in the more recent Sigma versions.  Although it
>> is nicer, I didn't think it was worth the trouble to support both
>> methods since the older method should work on all chips.
>
> Well, from my perspective, the older method does not work on newer
> chips :-)

It does work on tango4.

What does the tango5 do if you flood it with packets faster than the
kernel can keep up with?  That would make it hit the end of the rx
chain, which is apparently what makes it miserable with the current dma
stop code.

-- 
Måns Rullgård