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

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Date:	Tue, 10 Jun 2014 11:52:06 +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> 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; }
>
>That is a strange return statement - why += ??
>
>You could also check the sign of 'entries'.
>Say:
>	return entries > 0 ? entries : entries + fep->tx_ring_size;
>
Agree your suggestion.

>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. 

Thanks,
Andy
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