Re: [PATCH V4 net-next 3/3] tun: rx batching

On Mon, Jan 09, 2017 at 10:39:55AM +0800, Jason Wang wrote:

On 2017年01月07日 03:47, Michael S. Tsirkin wrote:

quoted

quoted

+static int tun_get_coalesce(struct net_device *dev,
+			    struct ethtool_coalesce *ec)
+{
+	struct tun_struct *tun = netdev_priv(dev);
+
+	ec->rx_max_coalesced_frames = tun->rx_batched;
+
+	return 0;
+}
+
+static int tun_set_coalesce(struct net_device *dev,
+			    struct ethtool_coalesce *ec)
+{
+	struct tun_struct *tun = netdev_priv(dev);
+
+	if (ec->rx_max_coalesced_frames > NAPI_POLL_WEIGHT)
+		return -EINVAL;

So what should userspace do? Keep trying until it succeeds?
I think it's better to just use NAPI_POLL_WEIGHT instead and DTRT here.

Well, looking at how set_coalesce is implemented in other drivers, -EINVAL
is usually used when user give a value that exceeds the limitation. For
tuntap, what missed here is probably just a documentation for coalescing in
tuntap.txt. (Or extend ethtool to return the max value). This seems much
better than silently reduce the value to the limitation.

Thanks

I don't think it's better, it's mostly that
1. there's a hardware limit so it does not change much
2. default is enabled and no one bothers changing

I don't see how will tuntap.txt help if we want to change it
in the future.

-- 
MST
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