Re: [PATCH net-next 10/10] net/mlx5e: Use the 'num_doorbells' devlink param

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Sep 10, 2025 at 09:16:40AM -0700, Stanislav Fomichev wrote:

> > +   * - ``num_doorbells``
> > +     - driverinit
> > +     - This controls the number of channel doorbells used by the netdev. In all
> > +       cases, an additional doorbell is allocated and used for non-channel
> > +       communication (e.g. for PTP, HWS, etc.). Supported values are:
> > +       - 0: No channel-specific doorbells, use the global one for everything.
> > +       - [1, max_num_channels]: Spread netdev channels equally across these
> > +         doorbells.
> 
> Do you have any guidance on this number? Why would the user want
> `num_doorbells < num_doorbells` vs `num_doorbells == num_channels`?

I expect it to be common that most deployment should continue to use
the historical value of num_doorbells = 0.

Certain systems with troubled CPUs will need to increase this, I don't
know if we yet fully understand what values these CPUs will need.

Nor do I think I'm permitted to say what CPUs are troubled :\

> IOW, why not allocate the same number of doorbells as the number of
> channels and do it unconditionally without devlink param? Are extra
> doorbells causing any overhead in the non-contended case?

It has a cost that should be minimized to not harm the current
majority of users.

Jason