[RFC/RFT PATCH net-next v3 0/2] net: Split ndo_set_rx_mode into snapshot and deferred write

[I Viswanath <viswanathiyyappan@gmail.com>]

This is an implementation of the idea provided by Jakub here

https://lore.kernel.org/netdev/20250923163727.5e97abdb@kernel.org/

ndo_set_rx_mode is problematic because it cannot sleep.

To address this, this series proposes dividing existing set_rx_mode
implementations into set_rx_mode and write_rx_config

The new set_rx_mode will be responsible for updating the rx_config
snapshot which will be used by ndo_write_rx_config to update the hardware

In brief, The callback implementations should look something like:

set_rx_mode():
    prepare_rx_config();
    update_snapshot();

write_rx_config():
    read_snapshot();
    do_io();

write_rx_config() is called from a work item making it sleepable
during the do_io() section.

This model should work correctly if the following conditions hold:

1. write_rx_config should use the rx_config set by the most recent
    call to set_rx_mode before its execution.

2. If a set_rx_mode call happens during execution of write_rx_config,
    write_rx_config should be rescheduled.

3. All calls to modify rx_mode should pass through the set_rx_mode +
    schedule write_rx_config execution flow.

1 and 2 are guaranteed because of the properties of work queues

Drivers need to ensure 3

ndo_write_rx_config has been implemented for 8139cp driver as proof of
concept

To use this model, a driver needs to implement the
ndo_write_rx_config callback, have a member rx_config in
the priv struct and replace all calls to set rx mode with
schedule_and_set_rx_mode();
---
v1:
Link: https://lore.kernel.org/netdev/20251020134857.5820-1-viswanathiyyappan@gmail.com/

v2:
- Exported set_and_schedule_rx_config as a symbol for use in modules
- Fixed incorrect cleanup for the case of rx_work alloc failing in alloc_netdev_mqs
- Removed the locked version (cp_set_rx_mode) and renamed __cp_set_rx_mode to cp_set_rx_mode
Link: https://lore.kernel.org/netdev/20251026175445.1519537-1-viswanathiyyappan@gmail.com/

v3:
- Added RFT tag
- Corrected mangled patch

I Viswanath (2):
  net: Add ndo_write_rx_config and helper structs and functions:
  net: ethernet: Implement ndo_write_rx_config callback for the 8139cp
    driver

 drivers/net/ethernet/realtek/8139cp.c | 78 ++++++++++++++++-----------
 include/linux/netdevice.h             | 38 ++++++++++++-
 net/core/dev.c                        | 53 ++++++++++++++++--
 3 files changed, 131 insertions(+), 38 deletions(-)
-- 
2.34.1