• Vladimir Oltean's avatar
    net: dsa: replace NETDEV_PRE_CHANGE_HWTSTAMP notifier with a stub · 5a178186
    Vladimir Oltean authored
    There was a sort of rush surrounding commit 88c0a6b5 ("net: create a
    netdev notifier for DSA to reject PTP on DSA master"), due to a desire
    to convert DSA's attempt to deny TX timestamping on a DSA master to
    something that doesn't block the kernel-wide API conversion from
    ndo_eth_ioctl() to ndo_hwtstamp_set().
    
    What was required was a mechanism that did not depend on ndo_eth_ioctl(),
    and what was provided was a mechanism that did not depend on
    ndo_eth_ioctl(), while at the same time introducing something that
    wasn't absolutely necessary - a new netdev notifier.
    
    There have been objections from Jakub Kicinski that using notifiers in
    general when they are not absolutely necessary creates complications to
    the control flow and difficulties to maintainers who look at the code.
    So there is a desire to not use notifiers.
    
    In addition to that, the notifier chain gets called even if there is no
    DSA in the system and no one is interested in applying any restriction.
    
    Take the model of udp_tunnel_nic_ops and introduce a stub mechanism,
    through which net/core/dev_ioctl.c can call into DSA even when
    CONFIG_NET_DSA=m.
    
    Compared to the code that existed prior to the notifier conversion, aka
    what was added in commits:
    - 4cfab356 ("net: dsa: Add wrappers for overloaded ndo_ops")
    - 3369afba ("net: Call into DSA netdevice_ops wrappers")
    
    this is different because we are not overloading any struct
    net_device_ops of the DSA master anymore, but rather, we are exposing a
    rather specific functionality which is orthogonal to which API is used
    to enable it - ndo_eth_ioctl() or ndo_hwtstamp_set().
    
    Also, what is similar is that both approaches use function pointers to
    get from built-in code to DSA.
    
    There is no point in replicating the function pointers towards
    __dsa_master_hwtstamp_validate() once for every CPU port (dev->dsa_ptr).
    Instead, it is sufficient to introduce a singleton struct dsa_stubs,
    built into the kernel, which contains a single function pointer to
    __dsa_master_hwtstamp_validate().
    
    I find this approach preferable to what we had originally, because
    dev->dsa_ptr->netdev_ops->ndo_do_ioctl() used to require going through
    struct dsa_port (dev->dsa_ptr), and so, this was incompatible with any
    attempts to add any data encapsulation and hide DSA data structures from
    the outside world.
    
    Link: https://lore.kernel.org/netdev/20230403083019.120b72fd@kernel.org/Signed-off-by: default avatarVladimir Oltean <vladimir.oltean@nxp.com>
    Reviewed-by: default avatarFlorian Fainelli <f.fainelli@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    5a178186
master.c 11.9 KB