netdev - Re: [PATCH 2/2] net: davinci_cpdma: reduce time holding chan->lock in cpdma_chan

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Date:	Wed, 27 Jul 2016 17:08:37 +0300
From:	Grygorii Strashko <grygorii.strashko@...com>
To:	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
CC:	Mugunthan V N <mugunthanvnm@...com>, <linux-omap@...r.kernel.org>,
	<netdev@...r.kernel.org>, <kernel@...gutronix.de>,
	Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
Subject: Re: [PATCH 2/2] net: davinci_cpdma: reduce time holding chan->lock in
 cpdma_chan_submit

On 07/27/2016 10:12 AM, Uwe Kleine-König wrote:
> Hello,
> 
> On Tue, Jul 26, 2016 at 05:25:58PM +0300, Grygorii Strashko wrote:
>> On 07/26/2016 03:02 PM, Uwe Kleine-König wrote:
>>> Allocating and preparing a dma descriptor doesn't need to happen under
>>> the channel's lock. So do this before taking the channel's lock. The only
>>> down side is that the dma descriptor might be allocated even though the
>>> channel is about to be stopped. This is unlikely though.
>>>
>>> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
>>> ---
>>>  drivers/net/ethernet/ti/davinci_cpdma.c | 38 +++++++++++++++++----------------
>>>  1 file changed, 20 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/ti/davinci_cpdma.c b/drivers/net/ethernet/ti/davinci_cpdma.c
>>> index 5ffa04a306c6..ba3462707ae3 100644
>>> --- a/drivers/net/ethernet/ti/davinci_cpdma.c
>>> +++ b/drivers/net/ethernet/ti/davinci_cpdma.c
>>> @@ -542,24 +542,10 @@ int cpdma_chan_submit(struct cpdma_chan *chan, void *token, void *data,
>>>  	u32				mode;
>>>  	int				ret = 0;
>>>  
>>> -	spin_lock_irqsave(&chan->lock, flags);
>>> -
>>> -	if (chan->state == CPDMA_STATE_TEARDOWN) {
>>> -		ret = -EINVAL;
>>> -		goto unlock_ret;
>>> -	}
>>> -
>>> -	if (chan->count >= chan->desc_num)	{
>>> -		chan->stats.desc_alloc_fail++;
>>> -		ret = -ENOMEM;
>>> -		goto unlock_ret;
>>> -	}
>>
>> I'm not sure this is right thing to do. This check is expected to be strict
>> and means "channel has exhausted the available descriptors, so further descs allocation does not allowed".
> 
> I developed this patch basing on a 4.4 kernel which doesn't have
> 742fb20fd4c7 ("net: ethernet: ti: cpdma: switch to use genalloc"). There
> my patch is more obviously correct. As currently chan->count is
> protected by chan->lock we must hold the lock for this check. If a
> failing check means we must not call cpdma_desc_alloc in the first
> place, that's bad.

Yes. That's intention of this check :(
Now it'll work as following for two (rx/tx) channels, as example
RX desc_num = 16 (max allowed number of descriptors)
TX desc_num = 16 (max allowed number of descriptors)
and with current code number of allocated descriptors will never exceed 16.

with your change, in corner case when TX channel's already utilized 16 descriptors the
following will happen:
cpdma_chan_submit()
 - cpdma_desc_alloc() -  will allocate 17th desc
 - lock
 - check for chan->count - fail
 - unlock
 - cpdma_desc_free() 

so your patch will add additional desc_alloc/desc_free in the above corner case
and that's what i'm worry about (TEARDOWN seems ok) especially taking into account
further multi-queue feature development.

Above corner case seems might happen very rare, because of the guard check in cpsw_ndo_start_xmit(), 
but it could.

> 
> But I'm not sure this is the case here. After all cpdma_desc_alloc
> doesn't do anything relevant for the hardware, right?

Right.

Thanks. I'd try to do some measurement also.
-- 
regards,
-grygorii