Re: [RFC PATCH net-next 2/4] net: dsa: remove the "dsa_to_port in a loop" antipattern from the core

On Tue, Aug 10, 2021 at 02:41:07AM -0700, Florian Fainelli wrote:

On 8/9/2021 8:33 PM, DENG Qingfang wrote:

quoted

On Mon, Aug 09, 2021 at 10:03:18PM +0300, Vladimir Oltean wrote:

quoted

Ever since Vivien's conversion of the ds->ports array into a dst->ports
list, and the introduction of dsa_to_port, iterations through the ports
of a switch became quadratic whenever dsa_to_port was needed.

So, what is the benefit of a linked list here? Do we allow users to
insert/delete a dsa_port at runtime? If not, how about using a
dynamically allocated array instead?

The goal was to flatten the space while doing cross switch operations, which
would have otherwise required iterating over dsa_switch instances within a
dsa_switch_tree, and then over dsa_port within each dsa_switch.

To expand on that: technically dsa_port_touch() _does_ happen at
runtime, since multiple switches in a cross-chip tree probe
asynchronously. To use a dynamically allocated array would mean to
preallocate the sum of all DSA switch ports' worth of memory, and to
preallocate an index for each DSA switch within that single array.
Overall a list is simpler.