`swadm link serdes set tx-eq` should require at least one gain value

[bnaecker]

See:

dendrite/swadm/src/link.rs

Lines 214 to 233 in b3cdd09

	/// Update the tx equalization settings for this port. Only the main setting is
	/// required. All others will default to 0. Note: to set a negative value,
	/// you must use the "=" option syntax. e.g., "--pre1=-1"
	#[clap(visible_alias = "txeq")]
	TxEq {
	/// The link path, specified as `switch_port/link`.
	///
	/// For example `rear0/0` is the first link on the rear0 switch port.
	link_path: LinkPath,
	#[clap(long)]
	pre2: Option<i32>,
	#[clap(long)]
	pre1: Option<i32>,
	#[clap(long)]
	main: Option<i32>,
	#[clap(long)]
	post1: Option<i32>,
	#[clap(long)]
	post2: Option<i32>,
	},

The comment says that swadm link serdes set tx-eq requires at least one value for the gains. That's a good idea, but the code actually lets you set none of them, in which case they're all defaulted to zero. We should disallow that, and require at least one.

[rcgoodfellow]

What about when the user wants to explicitly set everything to None to pick up the defaults?

[bnaecker]

That sounds useful too. What happens today though is all the parameters that are omitted are set to 0. That behavior definitely seems unhelpful:

root@oxz_switch1:~# swadm link serdes get tx-eq qsfp20/0
           lane 0     lane 1     lane 2     lane 3
pre2      0 (  0)    0 (  0)    0 (  0)    0 (  0)
pre1      0 (  0)    0 (  0)    0 (  0)    0 (  0)
main      7 (  7)    7 (  7)    7 (  7)    7 (  7)
post1     0 (  0)    0 (  0)    0 (  0)    0 (  0)
post2     0 (  0)    0 (  0)    0 (  0)    0 (  0)
root@oxz_switch1:~# swadm link serdes set tx-eq qsfp20/0
root@oxz_switch1:~# swadm link serdes get tx-eq qsfp20/0
           lane 0     lane 1     lane 2     lane 3
pre2      0 (  0)    0 (  0)    0 (  0)    0 (  0)
pre1      0 (  0)    0 (  0)    0 (  0)    0 (  0)
main      0 (  0)    0 (  0)    0 (  0)    0 (  0)
post1     0 (  0)    0 (  0)    0 (  0)    0 (  0)
post2     0 (  0)    0 (  0)    0 (  0)    0 (  0)

All these fields are optional, and I'm not sure what a value of None means. It could either mean "don't change it" or "set it to default". It seems better to explicitly enable the default values somehow through the API.