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

From: fugang.duan@freescale.com 

From: David Laight <redacted> Sent: Tuesday, June 10, 2014 7:11 PM

quoted

To: Duan Fugang-B38611; davem@davemloft.net
Cc: netdev@vger.kernel.org; ezequiel.garcia@free-electrons.com; Li Frank-
B20596; eric.dumazet@gmail.com
Subject: RE: [PATCH v4 5/6] net: fec: Add Scatter/gather support

From: Fugang Duan

quoted

Add Scatter/gather support for FEC.
This feature allows to improve outbound throughput performance.

...

quoted

+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; }

...

quoted

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