netdev - Re: [PATCH v3] ethernet/arc/arc_emac

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Date:	Fri, 14 Jun 2013 21:27:04 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	Joe Perches <joe@...ches.com>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Mischa Jonker <Mischa.Jonker@...opsys.com>,
	Arnd Bergmann <arnd@...db.de>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"David S. Miller" <davem@...emloft.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>
Subject: Re: [PATCH v3] ethernet/arc/arc_emac - Add new driver

Alexey Brodkin <Alexey.Brodkin@...opsys.com> :
> On 06/14/2013 02:20 AM, Francois Romieu wrote:
[...]
> >> +struct arc_emac_priv {
> >> +	struct net_device_stats stats;
> >> +	unsigned int clock_frequency;
> >> +	unsigned int max_speed;
> >> +
> >> +	/* Pointers to BD rings - CPU side */
> >> +	struct arc_emac_bd_t *rxbd;
> >
> > There does not seem to be much need for rxbd->data.
> 
> Could you please clarify this comment? Not clear what do you mean.

Rx and Tx use the same struct but they don't work the same.
They could/should use differents struct.

[...]
> > The descriptor entry is left unchanged. Afaiu the driver will move to the
> > next descriptor and crash on dereferencing NULL (rx_buff->)skb next time
> > it wraps.
> >
> > I suggest avoiding holes: don't netif_receive_skb if you can't alloc a new
> > skb.
> 
> Frankly I cannot understand how "don't netif_receive_skb" for one of the 
> received packets helps to prevent crash on the next iteration?
> And I don't see a way to return any error state from NAPI poll handler.
> Could you please clarify your idea?

The driver assigns the otherwise-netif_received skb to the current rx
descriptor as if it was a newly allocated one. The driver increases
stats.rx_dropped. The rx descriptor ring doesn't ever exhibit a hole.

[...]
> >> +static int arc_emac_tx(struct sk_buff *skb, struct net_device *ndev)
> >> +{
> >> +	struct arc_emac_priv *priv = netdev_priv(ndev);
> >> +	unsigned int info, len, *txbd_curr = &priv->txbd_curr;
> >> +	dma_addr_t addr;
> >> +	char *pkt = skb->data;
> >> +
> >> +	len = max_t(unsigned int, ETH_ZLEN, skb->len);
> >
> > The device automatically pads, right ?
> 
> What do you mean here?

Does the device fill a smaller than 64 bytes packet with zeroes or may
the driver leak information ? The driver should use skb_pad if the
latter applies.

-- 
Ueimor
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