netdev - Re: [PATCH net-next v2 1/5] net: ipqess: introduce the Qualcomm IPQESS driver

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Date:   Tue, 17 May 2022 09:11:34 +0200
From:   Maxime Chevallier <maxime.chevallier@...tlin.com>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        Rob Herring <robh+dt@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "thomas.petazzoni@...tlin.com" <thomas.petazzoni@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Luka Perkov <luka.perkov@...tura.hr>,
        Robert Marko <robert.marko@...tura.hr>
Subject: Re: [PATCH net-next v2 1/5] net: ipqess: introduce the Qualcomm
 IPQESS driver

Hello Vlad,

On Sat, 14 May 2022 20:44:38 +0000
Vladimir Oltean <vladimir.oltean@....com> wrote:

> On Sat, May 14, 2022 at 05:06:52PM +0200, Maxime Chevallier wrote:
> > +/* locking is handled by the caller */
> > +static int ipqess_rx_buf_alloc_napi(struct ipqess_rx_ring *rx_ring)
> > +{
> > +	struct ipqess_buf *buf = &rx_ring->buf[rx_ring->head];
> > +
> > +	buf->skb = napi_alloc_skb(&rx_ring->napi_rx,
> > IPQESS_RX_HEAD_BUFF_SIZE);
> > +	if (!buf->skb)
> > +		return -ENOMEM;
> > +
> > +	return ipqess_rx_buf_prepare(buf, rx_ring);
> > +}
> > +
> > +static int ipqess_rx_buf_alloc(struct ipqess_rx_ring *rx_ring)
> > +{
> > +	struct ipqess_buf *buf = &rx_ring->buf[rx_ring->head];
> > +
> > +	buf->skb = netdev_alloc_skb_ip_align(rx_ring->ess->netdev,
> > +
> > IPQESS_RX_HEAD_BUFF_SIZE); +
> > +	if (!buf->skb)
> > +		return -ENOMEM;
> > +
> > +	return ipqess_rx_buf_prepare(buf, rx_ring);
> > +}
> > +
> > +static void ipqess_refill_work(struct work_struct *work)
> > +{
> > +	struct ipqess_rx_ring_refill *rx_refill =
> > container_of(work,
> > +		struct ipqess_rx_ring_refill, refill_work);
> > +	struct ipqess_rx_ring *rx_ring = rx_refill->rx_ring;
> > +	int refill = 0;
> > +
> > +	/* don't let this loop by accident. */
> > +	while (atomic_dec_and_test(&rx_ring->refill_count)) {
> > +		napi_disable(&rx_ring->napi_rx);
> > +		if (ipqess_rx_buf_alloc(rx_ring)) {
> > +			refill++;
> > +			dev_dbg(rx_ring->ppdev,
> > +				"Not all buffers were
> > reallocated");
> > +		}
> > +		napi_enable(&rx_ring->napi_rx);
> > +	}
> > +
> > +	if (atomic_add_return(refill, &rx_ring->refill_count))
> > +		schedule_work(&rx_refill->refill_work);
> > +}
> > +
> > +static int ipqess_rx_poll(struct ipqess_rx_ring *rx_ring, int
> > budget) +{  
> 
> > +	while (done < budget) {  
> 
> > +		num_desc += atomic_xchg(&rx_ring->refill_count, 0);
> > +		while (num_desc) {
> > +			if (ipqess_rx_buf_alloc_napi(rx_ring)) {
> > +				num_desc =
> > atomic_add_return(num_desc,
> > +
> > &rx_ring->refill_count);
> > +				if (num_desc >= ((4 *
> > IPQESS_RX_RING_SIZE + 6) / 7))  
> 
> DIV_ROUND_UP(IPQESS_RX_RING_SIZE * 4, 7)
> Also, why this number?

Ah this was from the original out-of-tree driver... I'll try to figure
out what's going on an replace that by some #define that would make
more sense.

> > +
> > schedule_work(&rx_ring->ess->rx_refill[rx_ring->ring_id].refill_work);
> > +				break;
> > +			}
> > +			num_desc--;
> > +		}
> > +	}
> > +
> > +	ipqess_w32(rx_ring->ess,
> > IPQESS_REG_RX_SW_CONS_IDX_Q(rx_ring->idx),
> > +		   rx_ring_tail);
> > +	rx_ring->tail = rx_ring_tail;
> > +
> > +	return done;
> > +}  
> 
> > +static void ipqess_rx_ring_free(struct ipqess *ess)
> > +{
> > +	int i;
> > +
> > +	for (i = 0; i < IPQESS_NETDEV_QUEUES; i++) {
> > +		int j;
> > +
> > +		atomic_set(&ess->rx_ring[i].refill_count, 0);
> > +		cancel_work_sync(&ess->rx_refill[i].refill_work);  
> 
> When refill_work is currently scheduled and executing the while loop,
> will refill_count underflow due to the possibility of calling
> atomic_dec_and_test(0)?

Good question, I'll double-check, you might be correct. Nice catch

> > +
> > +		for (j = 0; j < IPQESS_RX_RING_SIZE; j++) {
> > +			dma_unmap_single(&ess->pdev->dev,
> > +
> > ess->rx_ring[i].buf[j].dma,
> > +
> > ess->rx_ring[i].buf[j].length,
> > +					 DMA_FROM_DEVICE);
> > +
> > dev_kfree_skb_any(ess->rx_ring[i].buf[j].skb);
> > +		}
> > +	}
> > +  

Thanks,

Maxime