[RFC net-next PATCH 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_mode():
    read_snapshot();
    do_io();

write_rx_mode() 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();

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 | 67 ++++++++++++++++++---------
 include/linux/netdevice.h             | 38 ++++++++++++++-
 net/core/dev.c                        | 48 +++++++++++++++++--
 3 files changed, 127 insertions(+), 26 deletions(-)
---
I tested the correctness of this by comparing rx_config request in set_rx_config and
written values in write_rx_config.

The prints I used:

After new_config is set in set_rx_mode:

printk("Requested Values: rx_config: %8x, mc_filter[0]: %8x, mc_filter[1]: %8x", new_config.rx_mode, new_config.mc_filter[0], new_config.mc_filter[1]);

After the writes in write_rx_config:

printk("RxConfig: %8x", cpr32(RxConfig));
printk("MC Filter[0]: %8x, MC Filter[1]: %8x", cpr32(MAR0 + 0), cpr32(MAR0 + 4));

Is this sufficient testing for a proof of concept?

I picked 8139cp because I could emulate it on QEMU and it was somewhat easy to refactor.

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2.47.3