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

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Date:	Tue, 10 Jun 2014 12:02:42 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	"'fugang.duan@...escale.com'" <fugang.duan@...escale.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: 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



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