netdev - RE: [PATCH v4 5/6] net: fec: Add Scatter/gather support

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Date:	Wed, 11 Jun 2014 02:36:16 +0000
From:	"fugang.duan@...escale.com" <fugang.duan@...escale.com>
To:	David Laight <David.Laight@...LAB.COM>,
	"davem@...emloft.net" <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>,
	"Frank.Li@...escale.com" <Frank.Li@...escale.com>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>
Subject: RE: [PATCH v4 5/6] net: fec: Add Scatter/gather support

From: David Laight <David.Laight@...LAB.COM> Data: Tuesday, June 10, 2014 8:03 PM
>To: Duan Fugang-B38611; davem@...emloft.net
>Cc: netdev@...r.kernel.org; ezequiel.garcia@...e-electrons.com; Li Frank-
>B20596; eric.dumazet@...il.com
>Subject: RE: [PATCH v4 5/6] net: fec: Add Scatter/gather support
>
>From: fugang.duan@...escale.com
>> From: David Laight <David.Laight@...LAB.COM> Sent: Tuesday, June 10,
>> 2014 7:11 PM
>> >To: Duan Fugang-B38611; davem@...emloft.net
>> >Cc: netdev@...r.kernel.org; ezequiel.garcia@...e-electrons.com; Li
>> >Frank- B20596; eric.dumazet@...il.com
>> >Subject: RE: [PATCH v4 5/6] net: fec: Add Scatter/gather support
>> >
>> >From: Fugang Duan
>> >> Add Scatter/gather support for FEC.
>> >> This feature allows to improve outbound throughput performance.
>> >...
>> >> +static int fec_enet_get_free_txdesc_num(struct fec_enet_private
>> >> +*fep) {
>> >> +	int entries;
>> >> +
>> >> +	entries = ((const char *)fep->dirty_tx -
>> >> +			(const char *)fep->cur_tx) / fep->bufdesc_size;
>> >> +
>> >> +	return (fep->cur_tx >= fep->dirty_tx) ?
>> >> +		entries += fep->tx_ring_size : entries; }
>...
>> >Actually do you ever use the last entry?
>> >If you do then the full and empty conditions are not separated.
>
>> Don't use the last entry. When it is full condition, cur_tx is the last
>entry.
>> If (fep->cur_tx == fep->dirty_tx)
>> 	Empty
>> If (fep->cur_tx == fep->dirty_tx - 1)
>> 	Full
>>
>> Since the patch set introduces tx_stop_threshold and
>> tx_wake_threshold, it never uses the rest of entry while Free entry
>> number is less than tx_stop_threshold.
>
>In which case I'd expect fec_enet_get_free_txdesc_num() to return one less
>than it does currently.
>Maybe the check for 'enough space' is 'needed <
>fec_enet_get_free_txdesc_num()'
>(rather than <=)
>
>	David
>
Return space available, 0..size-1.  We always leave one free entry. Which is same as linux circ_buf.
static int fec_enet_get_free_txdesc_num(struct fec_enet_private *fep)
{
	int entries;

	entries = ((const char *)fep->dirty_tx - (const char *)fep->cur_tx) / fep->bufdesc_size - 1;

	return entries > 0 ? entries : entries + fep->tx_ring_size;
}

For SG,
entries_free = fec_enet_get_free_txdesc_num(fep);
=> stop queue:
if (entries_free < MAX_SKB_FRAGS + 1) 
     netif_stop_queue(ndev);
=>wake queue:
if (entries >= MAX_SKB_FRAGS + 1)
	netif_wake_queue(ndev);

Thanks,
Andy
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