• Vladimir Oltean's avatar
    net: dsa: Make deferred_xmit private to sja1105 · a68578c2
    Vladimir Oltean authored
    There are 3 things that are wrong with the DSA deferred xmit mechanism:
    
    1. Its introduction has made the DSA hotpath ever so slightly more
       inefficient for everybody, since DSA_SKB_CB(skb)->deferred_xmit needs
       to be initialized to false for every transmitted frame, in order to
       figure out whether the driver requested deferral or not (a very rare
       occasion, rare even for the only driver that does use this mechanism:
       sja1105). That was necessary to avoid kfree_skb from freeing the skb.
    
    2. Because L2 PTP is a link-local protocol like STP, it requires
       management routes and deferred xmit with this switch. But as opposed
       to STP, the deferred work mechanism needs to schedule the packet
       rather quickly for the TX timstamp to be collected in time and sent
       to user space. But there is no provision for controlling the
       scheduling priority of this deferred xmit workqueue. Too bad this is
       a rather specific requirement for a feature that nobody else uses
       (more below).
    
    3. Perhaps most importantly, it makes the DSA core adhere a bit too
       much to the NXP company-wide policy "Innovate Where It Doesn't
       Matter". The sja1105 is probably the only DSA switch that requires
       some frames sent from the CPU to be routed to the slave port via an
       out-of-band configuration (register write) rather than in-band (DSA
       tag). And there are indeed very good reasons to not want to do that:
       if that out-of-band register is at the other end of a slow bus such
       as SPI, then you limit that Ethernet flow's throughput to effectively
       the throughput of the SPI bus. So hardware vendors should definitely
       not be encouraged to design this way. We do _not_ want more
       widespread use of this mechanism.
    
    Luckily we have a solution for each of the 3 issues:
    
    For 1, we can just remove that variable in the skb->cb and counteract
    the effect of kfree_skb with skb_get, much to the same effect. The
    advantage, of course, being that anybody who doesn't use deferred xmit
    doesn't need to do any extra operation in the hotpath.
    
    For 2, we can create a kernel thread for each port's deferred xmit work.
    If the user switch ports are named swp0, swp1, swp2, the kernel threads
    will be named swp0_xmit, swp1_xmit, swp2_xmit (there appears to be a 15
    character length limit on kernel thread names). With this, the user can
    change the scheduling priority with chrt $(pidof swp2_xmit).
    
    For 3, we can actually move the entire implementation to the sja1105
    driver.
    
    So this patch deletes the generic implementation from the DSA core and
    adds a new one, more adequate to the requirements of PTP TX
    timestamping, in sja1105_main.c.
    Suggested-by: default avatarFlorian Fainelli <f.fainelli@gmail.com>
    Signed-off-by: default avatarVladimir Oltean <olteanv@gmail.com>
    Reviewed-by: default avatarFlorian Fainelli <f.fainelli@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    a68578c2
dsa_priv.h 5.41 KB