When chan->direct_xmit is true, and no compressors are in use, PPP
prepends its header to a skb, and calls dev_queue_xmit directly. In this
mode the skb does not need to be linearized.
Enable NETIF_F_SG and NETIF_F_FRAGLIST, and add .ndo_fix_features()
callback to conditionally disable them if a linear skb is required.
This is required to support PPPoE GSO.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
---
RFC v2:
 Dynamically update netdev features with ndo_fix_features() callback.
 Link to RFC v1: https://lore.kernel.org/netdev/20250904021328.24329-1-dqfext@gmail.com/

 drivers/net/ppp/ppp_generic.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index f9f0f16c41d1..22e17f8fb61f 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -835,6 +835,10 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		ppp_unlock(ppp);
 		if (cflags & SC_CCP_OPEN)
 			ppp_ccp_closed(ppp);
+
+		rtnl_lock();
+		netdev_update_features(ppp->dev);
+		rtnl_unlock();
 		err = 0;
 		break;
 
@@ -1545,6 +1549,22 @@ ppp_get_stats64(struct net_device *dev, struct rtnl_link_stats64 *stats64)
 	dev_fetch_sw_netstats(stats64, dev->tstats);
 }
 
+static netdev_features_t
+ppp_fix_features(struct net_device *dev, netdev_features_t features)
+{
+	struct ppp *ppp = netdev_priv(dev);
+
+	ppp_lock(ppp);
+	/* Allow SG/FRAGLIST only when we have direct-xmit, and no compression
+	 * path that wants a linear skb.
+	 */
+	if (!(dev->priv_flags & IFF_NO_QUEUE) ||
+	    (ppp->flags & (SC_COMP_TCP | SC_CCP_OPEN | SC_CCP_UP)))
+		features &= ~(NETIF_F_SG | NETIF_F_FRAGLIST);
+	ppp_unlock(ppp);
+	return features;
+}
+
 static int ppp_dev_init(struct net_device *dev)
 {
 	struct ppp *ppp;
@@ -1619,6 +1639,7 @@ static const struct net_device_ops ppp_netdev_ops = {
 	.ndo_start_xmit  = ppp_start_xmit,
 	.ndo_siocdevprivate = ppp_net_siocdevprivate,
 	.ndo_get_stats64 = ppp_get_stats64,
+	.ndo_fix_features = ppp_fix_features,
 	.ndo_fill_forward_path = ppp_fill_forward_path,
 };
 
@@ -1641,6 +1662,8 @@ static void ppp_setup(struct net_device *dev)
 	dev->flags = IFF_POINTOPOINT | IFF_NOARP | IFF_MULTICAST;
 	dev->priv_destructor = ppp_dev_priv_destructor;
 	dev->pcpu_stat_type = NETDEV_PCPU_STAT_TSTATS;
+	dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST;
+	dev->features = dev->hw_features;
 	netif_keep_dst(dev);
 }
 
@@ -3537,6 +3560,12 @@ ppp_connect_channel(struct channel *pch, int unit)
 	spin_unlock(&pch->upl);
  out:
 	mutex_unlock(&pn->all_ppp_mutex);
+	if (ret == 0) {
+		rtnl_lock();
+		netdev_update_features(ppp->dev);
+		rtnl_unlock();
+	}
+
 	return ret;
 }
 
-- 
2.43.0