* [PATCH net-next] ppp: enable TX scatter-gather
@ 2025-09-12 9:59 Qingfang Deng
2025-09-16 1:10 ` Jakub Kicinski
0 siblings, 1 reply; 6+ messages in thread
From: Qingfang Deng @ 2025-09-12 9:59 UTC (permalink / raw)
To: Andrew Lunn, David S. Miller, Eric Dumazet, Jakub Kicinski,
Paolo Abeni, linux-ppp, netdev, linux-kernel
Cc: Felix Fietkau
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>
---
v1:
Remove the test for SC_CCP_OPEN and instead test for xc_state changes.
Link to RFC v2: https://lore.kernel.org/netdev/20250909012742.424771-1-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 | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index f9f0f16c41d1..1132159a8b92 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_xmit_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->xc_state ||
+ ppp->flags & (SC_COMP_TCP | SC_CCP_UP))
+ features &= ~(NETIF_F_SG | NETIF_F_FRAGLIST);
+ ppp_xmit_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->features = NETIF_F_SG | NETIF_F_FRAGLIST;
+ dev->hw_features = dev->features;
netif_keep_dst(dev);
}
@@ -3081,6 +3104,9 @@ ppp_set_compress(struct ppp *ppp, struct ppp_option_data *data)
ocomp->comp_free(ostate);
module_put(ocomp->owner);
}
+ rtnl_lock();
+ netdev_update_features(ppp->dev);
+ rtnl_unlock();
err = 0;
} else
module_put(cp->owner);
@@ -3537,6 +3563,12 @@ 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
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [PATCH net-next] ppp: enable TX scatter-gather
2025-09-12 9:59 [PATCH net-next] ppp: enable TX scatter-gather Qingfang Deng
@ 2025-09-16 1:10 ` Jakub Kicinski
2025-09-16 2:57 ` Qingfang Deng
0 siblings, 1 reply; 6+ messages in thread
From: Jakub Kicinski @ 2025-09-16 1:10 UTC (permalink / raw)
To: Qingfang Deng
Cc: Andrew Lunn, David S. Miller, Eric Dumazet, Paolo Abeni,
linux-ppp, netdev, linux-kernel, Felix Fietkau
On Fri, 12 Sep 2025 17:59:27 +0800 Qingfang Deng wrote:
> 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.
Seems a bit racy. We can't netdev_update_features() under the spin lock
so there's going to be a window of time where datapath will see new
state but netdev flags won't be cleared, yet?
We either need to add a explicit linearization check in the xmit path,
or always reset the flags to disabled before we start tweaking the
config and re-enable after config (tho the latter feels like a bit of
a hack).
--
pw-bot: cr
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH net-next] ppp: enable TX scatter-gather
2025-09-16 1:10 ` Jakub Kicinski
@ 2025-09-16 2:57 ` Qingfang Deng
2025-09-16 14:57 ` Jakub Kicinski
0 siblings, 1 reply; 6+ messages in thread
From: Qingfang Deng @ 2025-09-16 2:57 UTC (permalink / raw)
To: Jakub Kicinski
Cc: Andrew Lunn, David S. Miller, Eric Dumazet, Paolo Abeni,
linux-ppp, netdev, linux-kernel, Felix Fietkau
Hi Jakub,
On Tue, Sep 16, 2025 at 9:10 AM Jakub Kicinski <kuba@kernel.org> wrote:
> Seems a bit racy. We can't netdev_update_features() under the spin lock
> so there's going to be a window of time where datapath will see new
> state but netdev flags won't be cleared, yet?
>
> We either need to add a explicit linearization check in the xmit path,
> or always reset the flags to disabled before we start tweaking the
> config and re-enable after config (tho the latter feels like a bit of
> a hack).
Can I modify dev->features directly under the spin lock (without
.ndo_fix_features) ?
> --
> pw-bot: cr
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH net-next] ppp: enable TX scatter-gather
2025-09-16 2:57 ` Qingfang Deng
@ 2025-09-16 14:57 ` Jakub Kicinski
2025-09-17 11:00 ` Qingfang Deng
0 siblings, 1 reply; 6+ messages in thread
From: Jakub Kicinski @ 2025-09-16 14:57 UTC (permalink / raw)
To: Qingfang Deng
Cc: Andrew Lunn, David S. Miller, Eric Dumazet, Paolo Abeni,
linux-ppp, netdev, linux-kernel, Felix Fietkau
On Tue, 16 Sep 2025 10:57:49 +0800 Qingfang Deng wrote:
> Hi Jakub,
>
> On Tue, Sep 16, 2025 at 9:10 AM Jakub Kicinski <kuba@kernel.org> wrote:
> > Seems a bit racy. We can't netdev_update_features() under the spin lock
> > so there's going to be a window of time where datapath will see new
> > state but netdev flags won't be cleared, yet?
> >
> > We either need to add a explicit linearization check in the xmit path,
> > or always reset the flags to disabled before we start tweaking the
> > config and re-enable after config (tho the latter feels like a bit of
> > a hack).
>
> Can I modify dev->features directly under the spin lock (without
> .ndo_fix_features) ?
Hm, I'm not aware of a reason not to. You definitely need to hold
rtnl_lock, and call netdev_update_features() after.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH net-next] ppp: enable TX scatter-gather
2025-09-16 14:57 ` Jakub Kicinski
@ 2025-09-17 11:00 ` Qingfang Deng
2025-09-17 21:14 ` Jakub Kicinski
0 siblings, 1 reply; 6+ messages in thread
From: Qingfang Deng @ 2025-09-17 11:00 UTC (permalink / raw)
To: Jakub Kicinski
Cc: Andrew Lunn, David S. Miller, Eric Dumazet, Paolo Abeni,
linux-ppp, netdev, linux-kernel, Felix Fietkau
On Tue, Sep 16, 2025 at 10:57 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 16 Sep 2025 10:57:49 +0800 Qingfang Deng wrote:
> > Can I modify dev->features directly under the spin lock (without
> > .ndo_fix_features) ?
>
> Hm, I'm not aware of a reason not to. You definitely need to hold
> rtnl_lock, and call netdev_update_features() after.
Will the modification race against __netdev_update_features(), where
dev->features is assigned a new value?
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH net-next] ppp: enable TX scatter-gather
2025-09-17 11:00 ` Qingfang Deng
@ 2025-09-17 21:14 ` Jakub Kicinski
0 siblings, 0 replies; 6+ messages in thread
From: Jakub Kicinski @ 2025-09-17 21:14 UTC (permalink / raw)
To: Qingfang Deng
Cc: Andrew Lunn, David S. Miller, Eric Dumazet, Paolo Abeni,
linux-ppp, netdev, linux-kernel, Felix Fietkau
On Wed, 17 Sep 2025 19:00:16 +0800 Qingfang Deng wrote:
> On Tue, Sep 16, 2025 at 10:57 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Tue, 16 Sep 2025 10:57:49 +0800 Qingfang Deng wrote:
> > > Can I modify dev->features directly under the spin lock (without
> > > .ndo_fix_features) ?
> >
> > Hm, I'm not aware of a reason not to. You definitely need to hold
> > rtnl_lock, and call netdev_update_features() after.
>
> Will the modification race against __netdev_update_features(), where
> dev->features is assigned a new value?
Shouldn't race if we're holding rtnl_lock when we make the modification
and until we call netdev_update_features()? I'm just spitballing tho,
haven't studied the code.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2025-09-12 9:59 [PATCH net-next] ppp: enable TX scatter-gather Qingfang Deng
2025-09-16 1:10 ` Jakub Kicinski
2025-09-16 2:57 ` Qingfang Deng
2025-09-16 14:57 ` Jakub Kicinski
2025-09-17 11:00 ` Qingfang Deng
2025-09-17 21:14 ` Jakub Kicinski
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