netdev - Re: [PATCH] bgmac: driver for GBit MAC core on BCMA bus

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Date:	Sat, 5 Jan 2013 19:28:50 +0100
From:	Rafał Miłecki <zajec5@...il.com>
To:	Francois Romieu <romieu@...zoreil.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] bgmac: driver for GBit MAC core on BCMA bus

2013/1/5 Francois Romieu <romieu@...zoreil.com>:
> Rafał Miłecki <zajec5@...il.com> :
> [...]
>> We really need some good explanation for NETDEV_TX_BUSY somewhere
>> (documentation!).
>
> It is already explained in Documentation/networking/driver.txt:
> <snip>
> 1) The ndo_start_xmit method must not return NETDEV_TX_BUSY under
>    any normal circumstances.  It is considered a hard error unless
>    there is no way your device can tell ahead of time when it's
>    transmit function will become busy.
>
>    Instead it must maintain the queue properly.  For example,
>    for a driver implementing scatter-gather this means:
> </snip>
>
> Please compare your V3:bgmac_dma_tx_add and the sample code after the
> aforementioned paragraph.
>
>> I've checked some drivers and:
>>
>> 1) b44.c
>> In case of:
>> > if (unlikely(TX_BUFFS_AVAIL(bp) < 1)) {
>> it does netif_stop_queue and returns NETDEV_TX_BUSY
>
> The driver labels it as bug checking and it stops queueing when need
> arises at the end of the xmit function.
>
>> 2) sky2.c
>> In case of:
>> > if (unlikely(tx_avail(sky2) < tx_le_req(skb)))
>> it just returns NETDEV_TX_BUSY
>
> Same thing as above. Less documented though.
>
>> 3) jme.2
>> In case of:
>> > if (unlikely(idx < 0)) {
>> it does netif_stop_queue and returns NETDEV_TX_BUSY
>
> The driver labels it as bug checking. It stops queueing when need
> arises at the end of the xmit function (see jme_stop_queue_if_full).
>
>> 4) de2104x.c
>> In case of:
>> > if (tx_free == 0) {
>> it does netif_stop_queue and returns NETDEV_TX_BUSY
>
> It stops queueing at the end of the start_xmit handler as well.
>
>> So there are many drivers doing the same mistake I did.
>
> After a packet has been handed to hardware, each of these drivers
> checks if queueing should be disabled before returning from start_xmit.
>
> V3 bgmac doesn't.

How did I miss that?! Thanks for pointing that out!

-- 
Rafał
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