Re: [PATCH net-next 5/7] Driver: Vmxnet3: Add support for get_coalesce, set_coalesce ethtool operations

[Shrikrishna Khare <skhare@vmware.com>]

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On Sat, 7 May 2016, Ben Hutchings wrote:

> On Fri, 2016-05-06 at 16:12 -0700, Shrikrishna Khare wrote:
> [...]
> > +static int
> > +vmxnet3_set_coalesce(struct net_device *netdev, struct ethtool_coalesce *ec)
> > +{
> [...]
> > +	switch (ec->rx_coalesce_usecs) {
> > +	case VMXNET3_COALESCE_DEFAULT:
> > +	case VMXNET3_COALESCE_DISABLED:
> > +	case VMXNET3_COALESCE_ADAPT:
> > +		if (ec->tx_max_coalesced_frames ||
> > +		    ec->tx_max_coalesced_frames_irq ||
> > +		    ec->rx_max_coalesced_frames_irq) {
> > +			return -EINVAL;
> > +		}
> > +		memset(adapter->coal_conf, 0, sizeof(*adapter->coal_conf));
> > +		adapter->coal_conf->coalMode = ec->rx_coalesce_usecs;
> > +		break;
> > +	case VMXNET3_COALESCE_STATIC:
> [...]
> 
> I don't want to see drivers introducing magic values for fields that
> are denominated in microseconds (especially not for 0, which is the
> correct way to specify 'no coalescing').  If the current
> ethtool_coalesce structure is inadequate, propose an extension.

For vmxnet3, we need an ethtool mechanism to indicate coalescing mode to 
the device.

Would a patch that maps 0 to 'no coalescing' be acceptable? That is:

rx-usecs = 0 -> coalescing disabled.
rx-usecs = 1 -> default (chosen by the device).
rx-usecs = 2 -> adaptive coalescing.
rx-usecs = 3 -> static coalescing.
all other rx-usecs values -> rate based coalescing where rx-usecs denotes 
rate.

Alternatively: I don't think new members could be added to struct 
ethtool_coalesce without breaking compatibility. Thus, I could extend 
coalescing as follows:
- new struct ethtool_coalesce_v2 with coalesce_mode (along with all the 
members of struct ethtool_coalesce).
- introduce new ETHTOOL_{G,S}COALESCE_V2 commands.
- extend userspace ethtool to invoke new commands.

Could you please advice?

Thanks,
Shri