netdev - Re: [PATCH net-next v3 06/10] net: dsa: Add support for deferred xmit

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Date:   Sun, 5 May 2019 10:03:24 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>, vivien.didelot@...il.com,
        andrew@...n.ch, davem@...emloft.net
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next v3 06/10] net: dsa: Add support for deferred xmit



On 5/5/2019 3:19 AM, Vladimir Oltean wrote:
> Some hardware needs to take work to get convinced to receive frames on
> the CPU port (such as the sja1105 which takes temporary L2 forwarding
> rules over SPI that last for a single frame). Such work needs a
> sleepable context, and because the regular .ndo_start_xmit is atomic,
> this cannot be done in the tagger. So introduce a generic DSA mechanism
> that sets up a transmit skb queue and a workqueue for deferred
> transmission.
> 
> The new driver callback (.port_deferred_xmit) is in dsa_switch and not
> in the tagger because the operations that require sleeping typically
> also involve interacting with the hardware, and not simply skb
> manipulations. Therefore having it there simplifies the structure a bit
> and makes it unnecessary to export functions from the driver to the
> tagger.
> 
> The driver is responsible of calling dsa_enqueue_skb which transfers it
> to the master netdevice. This is so that it has a chance of performing
> some more work afterwards, such as cleanup or TX timestamping.
> 
> To tell DSA that skb xmit deferral is required, I have thought about
> changing the return type of the tagger .xmit from struct sk_buff * into
> a enum dsa_tx_t that could potentially encode a DSA_XMIT_DEFER value.
> 
> But the trailer tagger is reallocating every skb on xmit and therefore
> making a valid use of the pointer return value. So instead of reworking
> the API in complicated ways, right now a boolean property in the newly
> introduced DSA_SKB_CB is set.
> 
> Signed-off-by: Vladimir Oltean <olteanv@...il.com>

Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
-- 
Florian