[RFC v1 04/22] net: clarify the meaning of netdev_config members

[Pavel Begunkov <asml.silence@gmail.com>]

From: Jakub Kicinski <kuba@kernel.org>

hds_thresh and hds_config are both inside struct netdev_config
but have quite different semantics. hds_config is the user config
with ternary semantics (on/off/unset). hds_thresh is a straight
up value, populated by the driver at init and only modified by
user space. We don't expect the drivers to have to pick a special
hds_thresh value based on other configuration.

The two approaches have different advantages and downsides.
hds_thresh ("direct value") gives core easy access to current
device settings, but there's no way to express whether the value
comes from the user. It also requires the initialization by
the driver.

hds_config ("user config values") tells us what user wanted, but
doesn't give us the current value in the core.

Try to explain this a bit in the comments, so at we make a conscious
choice for new values which semantics we expect.

Move the init inside ethtool_ringparam_get_cfg() to reflect the semantics.
Commit 216a61d33c07 ("net: ethtool: fix ethtool_ringparam_get_cfg()
returns a hds_thresh value always as 0.") added the setting for the
benefit of netdevsim which doesn't touch the value at all on get.
Again, this is just to clarify the intention, shouldn't cause any
functional change.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 include/net/netdev_queues.h | 19 +++++++++++++++++--
 net/ethtool/common.c        |  3 ++-
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/net/netdev_queues.h b/include/net/netdev_queues.h
index ba2eaf39089b..81df0794d84c 100644
--- a/include/net/netdev_queues.h
+++ b/include/net/netdev_queues.h
@@ -6,11 +6,26 @@
 
 /**
  * struct netdev_config - queue-related configuration for a netdev
- * @hds_thresh:		HDS Threshold value.
- * @hds_config:		HDS value from userspace.
  */
 struct netdev_config {
+	/* Direct value
+	 *
+	 * Driver default is expected to be fixed, and set in this struct
+	 * at init. From that point on user may change the value. There is
+	 * no explicit way to "unset" / restore driver default.
+	 */
+	/** @hds_thresh: HDS Threshold value (ETHTOOL_A_RINGS_HDS_THRESH).
+	 */
 	u32	hds_thresh;
+
+	/* User config values
+	 *
+	 * Contain user configuration. If "set" driver must obey.
+	 * If "unset" driver is free to decide, and may change its choice
+	 * as other parameters change.
+	 */
+	/** @hds_config: HDS enabled (ETHTOOL_A_RINGS_TCP_DATA_SPLIT).
+	 */
 	u8	hds_config;
 };
 
diff --git a/net/ethtool/common.c b/net/ethtool/common.c
index eb253e0fd61b..a87298f659f5 100644
--- a/net/ethtool/common.c
+++ b/net/ethtool/common.c
@@ -825,12 +825,13 @@ void ethtool_ringparam_get_cfg(struct net_device *dev,
 	memset(param, 0, sizeof(*param));
 	memset(kparam, 0, sizeof(*kparam));
 
+	kparam->hds_thresh = dev->cfg->hds_thresh;
+
 	param->cmd = ETHTOOL_GRINGPARAM;
 	dev->ethtool_ops->get_ringparam(dev, param, kparam, extack);
 
 	/* Driver gives us current state, we want to return current config */
 	kparam->tcp_data_split = dev->cfg->hds_config;
-	kparam->hds_thresh = dev->cfg->hds_thresh;
 }
 
 static void ethtool_init_tsinfo(struct kernel_ethtool_ts_info *info)
-- 
2.49.0