linux-kernel - Re: [PATCH v3] ethernet/arc/arc_emac

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Date:	Sat, 15 Jun 2013 08:51:39 +0000
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	Francois Romieu <romieu@...zoreil.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

On 06/14/2013 11:27 PM, Francois Romieu wrote:
> 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.

Structure "arc_emac_bd" represents a buffer descriptor that is used for 
intercommunication between EMAC and CPU for both Tx and Rx.

"info" field contains a number of flags (who's an owner of this BD now, 
if this buffer contains beginning and/or end of received packets, 
received packet length etc.) Those flags are used for both Tx and Rx.
At least some of them like "owner" and "length". In case of Tx "length" 
indicates size of memory buffer available for receiving next packet.

"data" field is a 32-bit pointer to memory buffer.
In case of Tx buffer contains data to send, in case of Rx buffer is used 
for receiving data from network.

If my comment doesn't make sense to you or I misunderstood your message 
could you please add more clarifications for your idea?

> [...]
>>> 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.

Makes sense, thanks!

> [...]
>>>> +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.

Done, thanks.

-Alexey

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