Re: [PATCH net-next] net: mana: Cache MANA_QUERY_LINK_CONFIG result to avoid repeated HWC queries

[Jacob Keller <jacob.e.keller@intel.com>]

On 5/28/2026 11:07 AM, Erni Sri Satya Vennela wrote:
> mana_query_link_cfg() sends an HWC command to firmware on every call,
> but the link speed and QoS values it returns only change when the
> driver explicitly calls mana_set_bw_clamp(). This function is called
> not only by userspace via ethtool get_link_ksettings, but also
> periodically by hv_netvsc through netvsc_get_link_ksettings and by
> the sysfs speed_show attribute via dev_attr_show, resulting in
> unnecessary HWC traffic every few minutes.
> 
> Add a link_cfg_error field to mana_port_context to cache the query
> result. The field uses three states: 1 (not yet queried, initial
> value set during mana_probe_port), 0 (success, speed/max_speed are
> valid), or a negative errno for permanent errors like -EOPNOTSUPP
> when the hardware does not support the command. Transient errors and
> qos_unconfigured responses are not cached so that subsequent calls
> will retry.
> 
> To prevent a concurrent mana_set_bw_clamp() from racing with an
> in-flight query and publishing stale pre-clamp speed/max_speed,
> serialize the firmware transaction and the cache update under a new
> per-port mutex (link_cfg_mutex). The mutex covers both the HWC
> request and the subsequent stores in mana_query_link_cfg(), and the
> HWC request and invalidation in mana_set_bw_clamp(). With this lock
> held, two queries can no longer interleave their speed/max_speed
> stores, and an invalidation can no longer slip in between a query's
> response and its publish.
> 
> Invalidate the cache inside mana_set_bw_clamp() on success, so all
> current and future callers that change the link configuration
> automatically trigger a fresh query on the next mana_query_link_cfg()
> call. Also reset link_cfg_error during resume in mana_probe() under
> link_cfg_mutex, so that any slow-path query already in flight cannot
> later store 0 and silently overwrite the post-resume invalidation.
> 
> Signed-off-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
> Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
> ---

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>