netdev - Re: net/macb: clear tx/rx completion flags in ISR

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Date:	Fri, 19 Apr 2013 17:21:20 +0800
From:	Hein Tibosch <hein_tibosch@...oo.es>
To:	Nicolas Ferre <nicolas.ferre@...el.com>
CC:	Steffen Trumtrar <s.trumtrar@...gutronix.de>,
	netdev@...r.kernel.org,
	Ludovic Desroches <ludovic.desroches@...el.com>
Subject: Re: net/macb: clear tx/rx completion flags in ISR

Hi,

On 4/19/2013 3:48 PM, Nicolas Ferre wrote:
> On 04/19/2013 09:30 AM, Steffen Trumtrar :
>> Hi Hein,
>>
>> On Fri, Apr 19, 2013 at 01:13:26PM +0800, Hein Tibosch wrote:
>>> Hi Steffen,
>>>
>>>> At least in the cadence IP core on the Xilinx Zynq SoC the TCOMP/RCOMP flags
>>>> are not auto-cleaned. As these flags are evaluated, they need to be cleaned.
>>> This patch does not work for at least the AVR32 platform. Both RCOMP/RCOMP
>>> are cleared by *reading* the ISR and writing them would be fatal.
>>>
>> :-(
>>
>>> Could you tell me the version of the macb of Xilinx Zynq?
>>>
>>>     u32 version = (macb_readl(bp, MID) & ((1 << MACB_REV_SIZE) - 1))
>>>             | MACB_GREGS_VERSION;
>>>
>>> On an AP7000 it reads as 0x0000010D
>>>
>> This gives me 0x00000119. The TRM says it is version r1p23.
>>
>>> I am thinking of making a patch like:
>>>
>>>     if (bp->version >= xxx)
>>>         macb_writel(bp, ISR, MACB_BIT(TCOMP));
>>>
>>>     if (bp->version >= xxx)
>>>         macb_writel(bp, ISR, MACB_BIT(RCOMP));
>>>
>>> which would make it work on both platforms.
> Well, keep in mind that it is the hot path: It can harm the performance
> if too much tests are performed...

Yes it's in the hot path, both tests will be done within an interrupt.

I just gave it a quick try:

---
 drivers/net/ethernet/cadence/macb.c |    8 ++++++--
 drivers/net/ethernet/cadence/macb.h |    1 +
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb.c b/drivers/net/ethernet/cadence/macb.c
index ed2cb13..fe31951 100644
--- a/drivers/net/ethernet/cadence/macb.c
+++ b/drivers/net/ethernet/cadence/macb.c
@@ -315,6 +315,8 @@ int macb_mii_init(struct macb *bp)
 	struct macb_platform_data *pdata;
 	int err = -ENXIO, i;
 
+	bp->ip_version = (macb_readl(bp, MID) & ((1 << MACB_REV_SIZE) - 1))
+			| MACB_GREGS_VERSION;
 	/* Enable management port */
 	macb_writel(bp, NCR, MACB_BIT(MPE));
 @@ -485,7 +487,8 @@ static void macb_tx_interrupt(struct macb *bp)
 	status = macb_readl(bp, TSR);
 	macb_writel(bp, TSR, status);
 
-	macb_writel(bp, ISR, MACB_BIT(TCOMP));
+	if (bp->ip_version == 0x00000119)
+		macb_writel(bp, ISR, MACB_BIT(TCOMP));
  	netdev_vdbg(bp->dev, "macb_tx_interrupt status = 0x%03lx\n",
 		(unsigned long)status);
@@ -738,7 +741,8 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id)
 			 * now.
 			 */
 			macb_writel(bp, IDR, MACB_RX_INT_FLAGS);
-			macb_writel(bp, ISR, MACB_BIT(RCOMP));
+			if (bp->ip_version == 0x00000119)
+				macb_writel(bp, ISR, MACB_BIT(RCOMP));
  			if (napi_schedule_prep(&bp->napi)) {
 				netdev_vdbg(bp->dev, "scheduling RX softirq\n");
diff --git a/drivers/net/ethernet/cadence/macb.h b/drivers/net/ethernet/cadence/macb.h
index 993d703..0fbb440 100644
--- a/drivers/net/ethernet/cadence/macb.h
+++ b/drivers/net/ethernet/cadence/macb.h
@@ -575,6 +575,7 @@ struct macb {
 	unsigned int 		duplex;
 
 	phy_interface_t		phy_interface;
+	unsigned		ip_version;
  	/* AT91RM9200 transmit */
 	struct sk_buff *skb;			/* holds skb until xmit interrupt completes */
--
1.7.10

On my AP7000 platforms, on a 100mbit LAN, I could not measure any drop
of performance using iperf:

Before the patch:
[  6]  0.0-10.1 sec  67.4 MBytes  56.1 Mbits/sec
[  7]  0.0-10.1 sec  65.5 MBytes  54.6 Mbits/sec
[  6]  0.0-10.1 sec  67.8 MBytes  56.4 Mbits/sec
[  7]  0.0-10.1 sec  65.1 MBytes  54.3 Mbits/sec

After the patch:
[  6]  0.0-10.1 sec  68.1 MBytes  56.8 Mbits/sec
[  7]  0.0-10.1 sec  66.5 MBytes  55.4 Mbits/sec
[  6]  0.0- 9.9 sec  67.5 MBytes  57.2 Mbits/sec
[  8]  0.0-10.1 sec  66.5 MBytes  55.5 Mbits/sec

It looks a bit faster, which is pure coincidence.
>
>> The documentation I have is a little bit confusing in that regard.
>> The cadence datasheet says, this register is R/W, the Xilinx datasheet says,
>> it is "normaly RO", but the programming guide explicitely mentions clearing
>> the bit by writing to it.

Steffen, did you really see it happen that TCOMP/RCOMP were not cleared by just
reading the ISR?
The Atmel manual says about each ISR field: 'Cleared on read'

>> It seems, that something like your patch is inevitable.
> I also had bad feedbacks concerning this patch. Maybe we should take
> more time to validate this change: event it is in net-next, maybe we
> should revert it for the moment...

Regards, Hein
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