RE: [PATCH v2 4/4] net: macb: Add hardware PTP support

From: Richard Cochran [mailto:richardcochran@gmail.com]
Sent: 6 czerwca 2017 20:34
To: Rafal Ozieblo <redacted>
Cc: David Miller <davem@davemloft.net>; nicolas.ferre@atmel.com;
netdev@vger.kernel.org; linux-kernel@vger.kernel.org;
devicetree@vger.kernel.org; linux-arm-kernel@lists.infradead.org;
harini.katakam@xilinx.com; andrei.pistirica@microchip.com
Subject: Re: [PATCH v2 4/4] net: macb: Add hardware PTP support

On Fri, Jun 02, 2017 at 03:28:10PM +0100, Rafal Ozieblo wrote:

quoted

+static void gem_ptp_clear_timer(struct macb *bp)
+{
+	bp->tsu_incr.ns = 0;
+	bp->tsu_incr.sub_ns = 0;

What is the point of this function?

Cleaning all bellow registers will stop hardware PTP clock.

quoted

+	gem_writel(bp, TISUBN, GEM_BF(SUBNSINCR, 0));
+	gem_writel(bp, TI, GEM_BF(NSINCR, 0));
+	gem_writel(bp, TA, 0);
+}

(...)

quoted

+int gem_ptp_txstamp(struct macb_queue *queue, struct sk_buff *skb,
+		    struct macb_dma_desc *desc)
+{
+	struct gem_tx_ts *tx_timestamp;
+	struct macb_dma_desc_ptp *desc_ptp;
+	unsigned long head = queue->tx_ts_head;
+	unsigned long tail = READ_ONCE(queue->tx_ts_tail);
+
+	if (!GEM_BFEXT(DMA_TXVALID, desc->ctrl))
+		return -EINVAL;
+
+	if (CIRC_SPACE(head, tail, PTP_TS_BUFFER_SIZE) == 0)
+		return -ENOMEM;
+
+	skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
+	desc_ptp = macb_ptp_desc(queue->bp, desc);
+	tx_timestamp = &queue->tx_timestamps[head];
+	tx_timestamp->skb = skb;
+	tx_timestamp->desc_ptp.ts_1 = desc_ptp->ts_1;
+	tx_timestamp->desc_ptp.ts_2 = desc_ptp->ts_2;
+	/* move head */
+	smp_store_release(&queue->tx_ts_head,
+			  (head + 1) & (PTP_TS_BUFFER_SIZE - 1));
+
+	schedule_work(&queue->tx_ts_task);

Since the time stamp is in the buffer descriptor, why delay the
delivery via the work item?

I put comment about that a few months ago:
https://patchwork.ozlabs.org/patch/705629/
Let me quote part about not doing it via worker:

" I think, you can not do it in that way. 
It will hold two locks. If you enable appropriate option in kernel (as far as I 
remember CONFIG_DEBUG_SPINLOCK) you will get a warning here.

Please look at following call-stack:

1. macb_interrupt()   // spin_lock(&bp->lock) is taken
2. macb_tx_interrupt()
3. macb_handle_txtstamp()
4. skb_tstamp_tx()
5. __skb_tstamp_tx()
6. skb_may_tx_timestamp()
7. read_lock_bh() // second lock is taken

I know that those are different locks and different types. But this could lead 
to deadlocks. This is the reason of warning I could see.
And this is the reason why I get timestamp in interrupt routine but pass it to 
skb outside interrupt (using circular buffer).

Please, refer to this:
https://lkml.org/lkml/2016/11/18/168

1. macb_tx_interrupt()
2. macb_tx_timestamp_add() and schedule_work(&queue->tx_timestamp_task)

Then, outside interrupt (without holding a lock) :
1. macb_tx_timestamp_flush()
2. macb_tstamp_tx()
3. skb_tstamp_tx()"

quoted

+	return 0;
+}

(...)

quoted

+void gem_ptp_remove(struct net_device *ndev)
+{
+	struct macb *bp = netdev_priv(ndev);
+
+	if (bp->ptp_clock)
+		ptp_clock_unregister(bp->ptp_clock);
+
+	gem_ptp_clear_timer(bp);

Why is this 'clear' needed?

To stop hardware PTP clock.

quoted

+	dev_info(&bp->pdev->dev, "%s ptp clock unregistered.\n",
+		 GEM_PTP_TIMER_NAME);
+}

Thanks,
Richard

I'll correct all other issues.

Regards,
Rafal