[net] net: mvneta: set real interrupt per packet for tx_done

From: Dmitri Epshtein <dima@marvell.com>

Commit aebea2ba0f74 ("net: mvneta: fix Tx interrupt delay") intended to
set coalescing threshold to a value guaranteeing interrupt generation
per each sent packet, so that buffers can be released with no delay.

In fact setting threshold to '1' was wrong, because it causes interrupt
every two packets. According to the documentation a reason behind it is
following - interrupt occurs once sent buffers counter reaches a value,
which is higher than one specified in MVNETA_TXQ_SIZE_REG(q). This
behavior was confirmed during tests. Also when testing the SoC working
as a NAS device, better performance was observed with int-per-packet,
as it strongly depends on the fact that all transmitted packets are
released immediately.

This commit enables NETA controller work in interrupt per sent packet mode
by setting coalescing threshold to 0.

Signed-off-by: Dmitri Epshtein <dima@marvell.com>
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Cc: <stable@vger.kernel.org> # v3.10+
Fixes aebea2ba0f74 ("net: mvneta: fix Tx interrupt delay")
---
 drivers/net/ethernet/marvell/mvneta.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Hi,

On Wed, Jul 06, 2016 at 04:18:58AM +0200, Marcin Wojtas wrote:
> From: Dmitri Epshtein <dima@marvell.com>
> 
> Commit aebea2ba0f74 ("net: mvneta: fix Tx interrupt delay") intended to
> set coalescing threshold to a value guaranteeing interrupt generation
> per each sent packet, so that buffers can be released with no delay.
> 
> In fact setting threshold to '1' was wrong, because it causes interrupt
> every two packets. According to the documentation a reason behind it is
> following - interrupt occurs once sent buffers counter reaches a value,
> which is higher than one specified in MVNETA_TXQ_SIZE_REG(q). This
> behavior was confirmed during tests. Also when testing the SoC working
> as a NAS device, better performance was observed with int-per-packet,
> as it strongly depends on the fact that all transmitted packets are
> released immediately.
> 
> This commit enables NETA controller work in interrupt per sent packet mode
> by setting coalescing threshold to 0.

We had a discussion about this in January 2015 and I thought I sent a patch
to change it but I can't find it, so I think it was only proposed to some
users for testing. I also remember that on more recent kernels by then
(>=3.13) we observed a slightly better performance with this value set to
zero.

Acked-by: Willy Tarreau <w@1wt.eu>

Willy

From: Marcin Wojtas <mw@semihalf.com>
Date: Wed,  6 Jul 2016 04:18:58 +0200

> From: Dmitri Epshtein <dima@marvell.com>
> 
> Commit aebea2ba0f74 ("net: mvneta: fix Tx interrupt delay") intended to
> set coalescing threshold to a value guaranteeing interrupt generation
> per each sent packet, so that buffers can be released with no delay.
> 
> In fact setting threshold to '1' was wrong, because it causes interrupt
> every two packets. According to the documentation a reason behind it is
> following - interrupt occurs once sent buffers counter reaches a value,
> which is higher than one specified in MVNETA_TXQ_SIZE_REG(q). This
> behavior was confirmed during tests. Also when testing the SoC working
> as a NAS device, better performance was observed with int-per-packet,
> as it strongly depends on the fact that all transmitted packets are
> released immediately.
> 
> This commit enables NETA controller work in interrupt per sent packet mode
> by setting coalescing threshold to 0.
> 
> Signed-off-by: Dmitri Epshtein <dima@marvell.com>
> Signed-off-by: Marcin Wojtas <mw@semihalf.com>
> Cc: <stable@vger.kernel.org> # v3.10+
> Fixes aebea2ba0f74 ("net: mvneta: fix Tx interrupt delay")

Applied, thanks.