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Date: Fri, 31 Jan 2020 16:09:31 +0100
From: Petr Oros <poros@...hat.com>
To: Heiner Kallweit <hkallweit1@...il.com>
Cc: netdev@...r.kernel.org, andrew@...n.ch, f.fainelli@...il.com,
ivecera@...hat.com
Subject: Re: [PATCH net v2] phy: avoid unnecessary link-up delay in polling
mode
Heiner Kallweit píše v St 29. 01. 2020 v 22:01 +0100:
> On 29.01.2020 13:19, Petr Oros wrote:
> > commit 93c0970493c71f ("net: phy: consider latched link-down status in
> > polling mode") removed double-read of latched link-state register for
> > polling mode from genphy_update_link(). This added extra ~1s delay into
> > sequence link down->up.
> > Following scenario:
> > - After boot link goes up
> > - phy_start() is called triggering an aneg restart, hence link goes
> > down and link-down info is latched.
> > - After aneg has finished link goes up. In phy_state_machine is checked
> > link state but it is latched "link is down". The state machine is
> > scheduled after one second and there is detected "link is up". This
> > extra delay can be avoided when we keep link-state register double read
> > in case when link was down previously.
> >
> > With this solution we don't miss a link-down event in polling mode and
> > link-up is faster.
> >
>
> I have a little problem to understand why it should be faster this way.
> Let's take an example: aneg takes 3.5s
> Current behavior:
>
> T0: aneg is started, link goes down, link-down status is latched
> (phydev->link is still 1)
> T0+1s: state machine runs, latched link-down is read,
> phydev->link goes down, state change PHY_UP to PHY_NOLINK
> T0+2s: state machine runs, up-to-date link-down is read
> T0+3s: state machine runs, up-to-date link-down is read
> T0+4s: state machine runs, aneg is finished, up-to-date link-up is read,
> phydev->link goes up, state change PHY_NOLINK to PHY_RUNNING
>
> Your patch changes the behavior of T0+1s only. So it should make a
> difference only if aneg takes less than 1s.
> Can you explain, based on the given example, how your change is
> supposed to improve this?
>
I see this behavior on real hw:
With patch:
T0+3s: state machine runs, up-to-date link-down is read
T0+4s: state machine runs, aneg is finished (BMSR_ANEGCOMPLETE==1),
first BMSR read: BMSR_ANEGCOMPLETE==1 and BMSR_LSTATUS==0,
second BMSR read: BMSR_ANEGCOMPLETE==1 and BMSR_LSTATUS==1,
phydev->link goes up, state change PHY_NOLINK to PHY_RUNNING
line: 1917 is first BMSR read
line: 1921 is second BMSR read
[ 24.124572] xgene-mii-rgmii:03: genphy_restart_aneg()
[ 24.132000] xgene-mii-rgmii:03: genphy_update_link(), line: 1895, link: 0
[ 24.139347] xgene-mii-rgmii:03: genphy_update_link(), line: 1917, status:
0x7949
[ 24.146783] xgene-mii-rgmii:03: genphy_update_link(), line: 1921, status:
0x7949
[ 24.154174] xgene-mii-rgmii:03: genphy_update_link(), line: 1927, link: 0
. supressed 3 same messages in T0+1,2,3s
[ 28.609822] xgene-mii-rgmii:03: genphy_update_link(), line: 1895, link: 0
[ 28.629906] xgene-mii-rgmii:03: genphy_update_link(), line: 1917, status:
0x7969
^^^^^^^^^^^^^^^ detected BMSR_ANEGCOMPLETE but not BMSR_LSTATUS
[ 28.644590] xgene-mii-rgmii:03: genphy_update_link(), line: 1921, status:
0x796d
^^^^^^^^^^^^^^^ here is detected BMSR_ANEGCOMPLETE and BMSR_LSTATUS
[ 28.658681] xgene-mii-rgmii:03: genphy_update_link(), line: 1927, link: 1
--------------------------------------------------------------------------------
---
Without patch:
T0+3s: state machine runs, up-to-date link-down is read
T0+4s: state machine runs, aneg is finished (BMSR_ANEGCOMPLETE==1),
here i read link-down (BMSR_LSTATUS==0),
T0+5s: state machine runs, aneg is finished (BMSR_ANEGCOMPLETE==1),
up-to-date link-up is read (BMSR_LSTATUS==1),
phydev->link goes up, state change PHY_NOLINK to PHY_RUNNING
line: 1917 is first BMSR read (status is zero because without patch it is readed
once)
line: 1921 is second BMSR read
[ 24.862702] xgene-mii-rgmii:03: 1768: genphy_restart_aneg
[ 24.869070] xgene-mii-rgmii:03: genphy_update_link(), line: 1895, link: 0
[ 24.876409] xgene-mii-rgmii:03: genphy_update_link(), line: 1917, status: 0x0
[ 24.885999] xgene-mii-rgmii:03: genphy_update_link(), line: 1921, status:
0x7949
[ 24.893401] xgene-mii-rgmii:03: genphy_update_link(), line: 1927, link: 0
. supressed 3 same messages in T0+1,2,3s
[ 29.319613] xgene-mii-rgmii:03: genphy_update_link(), line: 1895, link: 0
[ 29.326408] xgene-mii-rgmii:03: genphy_update_link(), line: 1917, status: 0x0
[ 29.333557] xgene-mii-rgmii:03: genphy_update_link(), line: 1921, status:
0x7969
^^^^^^^^^^^^^^^ detected BMSR_ANEGCOMPLETE but not BMSR_LSTATUS
[ 29.340923] xgene-mii-rgmii:03: genphy_update_link(), line: 1927, link: 0
[ 30.359713] xgene-mii-rgmii:03: genphy_update_link(), line: 1895, link: 0
[ 30.366507] xgene-mii-rgmii:03: genphy_update_link(), line: 1917, status: 0x0
[ 30.373650] xgene-mii-rgmii:03: genphy_update_link(), line: 1921, status:
0x796d
^^^^^^^^^^^^^^^ here is detected BMSR_ANEGCOMPLETE and BMSR_LSTATUS
[ 30.381016] xgene-mii-rgmii:03: genphy_update_link(), line: 1927, link: 1
I tried many variants and it is deterministic behavior. Without patch is delay
one second longer due to later detect link up after aneg finish
-Petr
> And on a side note: I wouldn't consider this change a fix, therefore
> it would be material for net-next that is closed at the moment.
>
> Heiner
>
> > Changes in v2:
> > - Fixed typos in phy_polling_mode() argument
> >
> > Fixes: 93c0970493c71f ("net: phy: consider latched link-down status in polling mode")
> > Signed-off-by: Petr Oros <poros@...hat.com>
> > ---
> > drivers/net/phy/phy-c45.c | 5 +++--
> > drivers/net/phy/phy_device.c | 5 +++--
> > 2 files changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/net/phy/phy-c45.c b/drivers/net/phy/phy-c45.c
> > index a1caeee1223617..bceb0dcdecbd61 100644
> > --- a/drivers/net/phy/phy-c45.c
> > +++ b/drivers/net/phy/phy-c45.c
> > @@ -239,9 +239,10 @@ int genphy_c45_read_link(struct phy_device *phydev)
> >
> > /* The link state is latched low so that momentary link
> > * drops can be detected. Do not double-read the status
> > - * in polling mode to detect such short link drops.
> > + * in polling mode to detect such short link drops except
> > + * the link was already down.
> > */
> > - if (!phy_polling_mode(phydev)) {
> > + if (!phy_polling_mode(phydev) || !phydev->link) {
> > val = phy_read_mmd(phydev, devad, MDIO_STAT1);
> > if (val < 0)
> > return val;
> > diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
> > index 6a5056e0ae7757..05417419c484fa 100644
> > --- a/drivers/net/phy/phy_device.c
> > +++ b/drivers/net/phy/phy_device.c
> > @@ -1930,9 +1930,10 @@ int genphy_update_link(struct phy_device *phydev)
> >
> > /* The link state is latched low so that momentary link
> > * drops can be detected. Do not double-read the status
> > - * in polling mode to detect such short link drops.
> > + * in polling mode to detect such short link drops except
> > + * the link was already down.
> > */
> > - if (!phy_polling_mode(phydev)) {
> > + if (!phy_polling_mode(phydev) || !phydev->link) {
> > status = phy_read(phydev, MII_BMSR);
> > if (status < 0)
> > return status;
> >
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