<feed xmlns='http://www.w3.org/2005/Atom'>
<title>linux/include, branch v4.20-rc4</title>
<subtitle>Linux Kernel (branches are rebased on master from time to time)</subtitle>
<id>https://sre.ring0.de/linux/atom?h=v4.20-rc4</id>
<link rel='self' href='https://sre.ring0.de/linux/atom?h=v4.20-rc4'/>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/'/>
<updated>2018-11-25T17:24:40Z</updated>
<entry>
<title>Merge tag 'dma-mapping-4.20-3' of git://git.infradead.org/users/hch/dma-mapping</title>
<updated>2018-11-25T17:24:40Z</updated>
<author>
<name>Linus Torvalds</name>
<email>torvalds@linux-foundation.org</email>
</author>
<published>2018-11-25T17:24:40Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=d6d460b89378b1bc6715574cdafd748ba59d5a27'/>
<id>urn:sha1:d6d460b89378b1bc6715574cdafd748ba59d5a27</id>
<content type='text'>
Pull dma-mapping fixes from Christoph Hellwig:
 "Two dma-direct / swiotlb regressions fixes:

   - zero is a valid physical address on some arm boards, we can't use
     it as the error value

   - don't try to cache flush the error return value (no matter what it
     is)"

* tag 'dma-mapping-4.20-3' of git://git.infradead.org/users/hch/dma-mapping:
  swiotlb: Skip cache maintenance on map error
  dma-direct: Make DIRECT_MAPPING_ERROR viable for SWIOTLB
</content>
</entry>
<entry>
<title>Merge tag 'xarray-4.20-rc4' of git://git.infradead.org/users/willy/linux-dax</title>
<updated>2018-11-25T02:44:01Z</updated>
<author>
<name>Linus Torvalds</name>
<email>torvalds@linux-foundation.org</email>
</author>
<published>2018-11-25T02:44:01Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=e2125dac22f2c9c66c412cd8e049a7305af59f73'/>
<id>urn:sha1:e2125dac22f2c9c66c412cd8e049a7305af59f73</id>
<content type='text'>
Pull XArray updates from Matthew Wilcox:
 "We found some bugs in the DAX conversion to XArray (and one bug which
  predated the XArray conversion). There were a couple of bugs in some
  of the higher-level functions, which aren't actually being called in
  today's kernel, but surfaced as a result of converting existing radix
  tree &amp; IDR users over to the XArray.

  Some of the other changes to how the higher-level APIs work were also
  motivated by converting various users; again, they're not in use in
  today's kernel, so changing them has a low probability of introducing
  a bug.

  Dan can still trigger a bug in the DAX code with hot-offline/online,
  and we're working on tracking that down"

* tag 'xarray-4.20-rc4' of git://git.infradead.org/users/willy/linux-dax:
  XArray tests: Add missing locking
  dax: Avoid losing wakeup in dax_lock_mapping_entry
  dax: Fix huge page faults
  dax: Fix dax_unlock_mapping_entry for PMD pages
  dax: Reinstate RCU protection of inode
  dax: Make sure the unlocking entry isn't locked
  dax: Remove optimisation from dax_lock_mapping_entry
  XArray tests: Correct some 64-bit assumptions
  XArray: Correct xa_store_range
  XArray: Fix Documentation
  XArray: Handle NULL pointers differently for allocation
  XArray: Unify xa_store and __xa_store
  XArray: Add xa_store_bh() and xa_store_irq()
  XArray: Turn xa_erase into an exported function
  XArray: Unify xa_cmpxchg and __xa_cmpxchg
  XArray: Regularise xa_reserve
  nilfs2: Use xa_erase_irq
  XArray: Export __xa_foo to non-GPL modules
  XArray: Fix xa_for_each with a single element at 0
</content>
</entry>
<entry>
<title>Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/hid/hid</title>
<updated>2018-11-24T20:58:47Z</updated>
<author>
<name>Linus Torvalds</name>
<email>torvalds@linux-foundation.org</email>
</author>
<published>2018-11-24T20:58:47Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=e195ca6cb6f21633e56322d5aa11ed59cdb22fb2'/>
<id>urn:sha1:e195ca6cb6f21633e56322d5aa11ed59cdb22fb2</id>
<content type='text'>
Pull HID fixes from Jiri Kosina:

 - revert of the high-resolution scrolling feature, as it breaks certain
   hardware due to incompatibilities between Logitech and Microsoft
   worlds. Peter Hutterer is working on a fixed implementation. Until
   that is finished, revert by Benjamin Tissoires.

 - revert of incorrect strncpy-&gt;strlcpy conversion in uhid, from David
   Herrmann

 - fix for buggy sendfile() implementation on uhid device node, from
   Eric Biggers

 - a few assorted device-ID specific quirks

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/hid/hid:
  Revert "Input: Add the `REL_WHEEL_HI_RES` event code"
  Revert "HID: input: Create a utility class for counting scroll events"
  Revert "HID: logitech: Add function to enable HID++ 1.0 "scrolling acceleration""
  Revert "HID: logitech: Enable high-resolution scrolling on Logitech mice"
  Revert "HID: logitech: Use LDJ_DEVICE macro for existing Logitech mice"
  Revert "HID: logitech: fix a used uninitialized GCC warning"
  Revert "HID: input: simplify/fix high-res scroll event handling"
  HID: Add quirk for Primax PIXART OEM mice
  HID: i2c-hid: Disable runtime PM for LG touchscreen
  HID: multitouch: Add pointstick support for Cirque Touchpad
  HID: steam: remove input device when a hid client is running.
  Revert "HID: uhid: use strlcpy() instead of strncpy()"
  HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
  HID: input: Ignore battery reported by Symbol DS4308
  HID: Add quirk for Microsoft PIXART OEM mouse
</content>
</entry>
<entry>
<title>Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net</title>
<updated>2018-11-24T17:19:38Z</updated>
<author>
<name>Linus Torvalds</name>
<email>torvalds@linux-foundation.org</email>
</author>
<published>2018-11-24T17:19:38Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=857fa628bbe93017c72ddd0d5304962a2608db07'/>
<id>urn:sha1:857fa628bbe93017c72ddd0d5304962a2608db07</id>
<content type='text'>
Pull networking fixes from David Miller:

 1) Need to take mutex in ath9k_add_interface(), from Dan Carpenter.

 2) Fix mt76 build without CONFIG_LEDS_CLASS, from Arnd Bergmann.

 3) Fix socket wmem accounting in SCTP, from Xin Long.

 4) Fix failed resume crash in ena driver, from Arthur Kiyanovski.

 5) qed driver passes bytes instead of bits into second arg of
    bitmap_weight(). From Denis Bolotin.

 6) Fix reset deadlock in ibmvnic, from Juliet Kim.

 7) skb_scrube_packet() needs to scrub the fwd marks too, from Petr
    Machata.

 8) Make sure older TCP stacks see enough dup ACKs, and avoid doing SACK
    compression during this period, from Eric Dumazet.

 9) Add atomicity to SMC protocol cursor handling, from Ursula Braun.

10) Don't leave dangling error pointer if bpf_prog_add() fails in
    thunderx driver, from Lorenzo Bianconi. Also, when we unmap TSO
    headers, set sq-&gt;tso_hdrs to NULL.

11) Fix race condition over state variables in act_police, from Davide
    Caratti.

12) Disable guest csum in the presence of XDP in virtio_net, from Jason
    Wang.

* git://git.kernel.org/pub/scm/linux/kernel/git/davem/net: (64 commits)
  net: gemini: Fix copy/paste error
  net: phy: mscc: fix deadlock in vsc85xx_default_config
  dt-bindings: dsa: Fix typo in "probed"
  net: thunderx: set tso_hdrs pointer to NULL in nicvf_free_snd_queue
  net: amd: add missing of_node_put()
  team: no need to do team_notify_peers or team_mcast_rejoin when disabling port
  virtio-net: fail XDP set if guest csum is negotiated
  virtio-net: disable guest csum during XDP set
  net/sched: act_police: add missing spinlock initialization
  net: don't keep lonely packets forever in the gro hash
  net/ipv6: re-do dad when interface has IFF_NOARP flag change
  packet: copy user buffers before orphan or clone
  ibmvnic: Update driver queues after change in ring size support
  ibmvnic: Fix RX queue buffer cleanup
  net: thunderx: set xdp_prog to NULL if bpf_prog_add fails
  net/dim: Update DIM start sample after each DIM iteration
  net: faraday: ftmac100: remove netif_running(netdev) check before disabling interrupts
  net/smc: use after free fix in smc_wr_tx_put_slot()
  net/smc: atomic SMCD cursor handling
  net/smc: add SMC-D shutdown signal
  ...
</content>
</entry>
<entry>
<title>packet: copy user buffers before orphan or clone</title>
<updated>2018-11-23T19:08:03Z</updated>
<author>
<name>Willem de Bruijn</name>
<email>willemb@google.com</email>
</author>
<published>2018-11-20T18:00:18Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=5cd8d46ea1562be80063f53c7c6a5f40224de623'/>
<id>urn:sha1:5cd8d46ea1562be80063f53c7c6a5f40224de623</id>
<content type='text'>
tpacket_snd sends packets with user pages linked into skb frags. It
notifies that pages can be reused when the skb is released by setting
skb-&gt;destructor to tpacket_destruct_skb.

This can cause data corruption if the skb is orphaned (e.g., on
transmit through veth) or cloned (e.g., on mirror to another psock).

Create a kernel-private copy of data in these cases, same as tun/tap
zerocopy transmission. Reuse that infrastructure: mark the skb as
SKBTX_ZEROCOPY_FRAG, which will trigger copy in skb_orphan_frags(_rx).

Unlike other zerocopy packets, do not set shinfo destructor_arg to
struct ubuf_info. tpacket_destruct_skb already uses that ptr to notify
when the original skb is released and a timestamp is recorded. Do not
change this timestamp behavior. The ubuf_info-&gt;callback is not needed
anyway, as no zerocopy notification is expected.

Mark destructor_arg as not-a-uarg by setting the lower bit to 1. The
resulting value is not a valid ubuf_info pointer, nor a valid
tpacket_snd frame address. Add skb_zcopy_.._nouarg helpers for this.

The fix relies on features introduced in commit 52267790ef52 ("sock:
add MSG_ZEROCOPY"), so can be backported as is only to 4.14.

Tested with from `./in_netns.sh ./txring_overwrite` from
http://github.com/wdebruij/kerneltools/tests

Fixes: 69e3c75f4d54 ("net: TX_RING and packet mmap")
Reported-by: Anand H. Krishnan &lt;anandhkrishnan@gmail.com&gt;
Signed-off-by: Willem de Bruijn &lt;willemb@google.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
</content>
</entry>
<entry>
<title>net/dim: Update DIM start sample after each DIM iteration</title>
<updated>2018-11-22T19:36:54Z</updated>
<author>
<name>Tal Gilboa</name>
<email>talgi@mellanox.com</email>
</author>
<published>2018-11-21T14:28:23Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=0211dda68a4f6531923a2f72d8e8959207f59fba'/>
<id>urn:sha1:0211dda68a4f6531923a2f72d8e8959207f59fba</id>
<content type='text'>
On every iteration of net_dim, the algorithm may choose to
check for the system state by comparing current data sample
with previous data sample. After each of these comparison,
regardless of the action taken, the sample used as baseline
is needed to be updated.

This patch fixes a bug that causes DIM to take wrong decisions,
due to never updating the baseline sample for comparison between
iterations. This way, DIM always compares current sample with
zeros.

Although this is a functional fix, it also improves and stabilizes
performance as the algorithm works properly now.

Performance:
Tested single UDP TX stream with pktgen:
samples/pktgen/pktgen_sample03_burst_single_flow.sh -i p4p2 -d 1.1.1.1
-m 24:8a:07:88:26:8b -f 3 -b 128

ConnectX-5 100GbE packet rate improved from 15-19Mpps to 19-20Mpps.
Also, toggling between profiles is less frequent with the fix.

Fixes: 8115b750dbcb ("net/dim: use struct net_dim_sample as arg to net_dim")
Signed-off-by: Tal Gilboa &lt;talgi@mellanox.com&gt;
Reviewed-by: Tariq Toukan &lt;tariqt@mellanox.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
</content>
</entry>
<entry>
<title>Merge tag 'usb-4.20-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb</title>
<updated>2018-11-22T16:39:29Z</updated>
<author>
<name>Linus Torvalds</name>
<email>torvalds@linux-foundation.org</email>
</author>
<published>2018-11-22T16:39:29Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=4cd731953d620b7e4e999a90d13db58b88c5e95b'/>
<id>urn:sha1:4cd731953d620b7e4e999a90d13db58b88c5e95b</id>
<content type='text'>
Pull USB fixes from Greg KH:
 "Here are a number of small USB fixes for 4.20-rc4.

  There's the usual xhci and dwc2/3 fixes as well as a few minor other
  issues resolved for problems that have been reported. Full details are
  in the shortlog.

  All have been in linux-next for a while with no reported issues"

* tag 'usb-4.20-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb:
  usb: cdc-acm: add entry for Hiro (Conexant) modem
  usb: xhci: Prevent bus suspend if a port connect change or polling state is detected
  usb: core: Fix hub port connection events lost
  usb: dwc3: gadget: fix ISOC TRB type on unaligned transfers
  Revert "usb: gadget: ffs: Fix BUG when userland exits with submitted AIO transfers"
  usb: dwc2: pci: Fix an error code in probe
  usb: dwc3: Fix NULL pointer exception in dwc3_pci_remove()
  xhci: Add quirk to workaround the errata seen on Cavium Thunder-X2 Soc
  usb: xhci: fix timeout for transition from RExit to U0
  usb: xhci: fix uninitialized completion when USB3 port got wrong status
  xhci: Add check for invalid byte size error when UAS devices are connected.
  xhci: handle port status events for removed USB3 hcd
  xhci: Fix leaking USB3 shared_hcd at xhci removal
  USB: misc: appledisplay: add 20" Apple Cinema Display
  USB: quirks: Add no-lpm quirk for Raydium touchscreens
  usb: quirks: Add delay-init quirk for Corsair K70 LUX RGB
  USB: Wait for extra delay time after USB_PORT_FEAT_RESET for quirky hub
  usb: dwc3: gadget: Properly check last unaligned/zero chain TRB
  usb: dwc3: core: Clean up ULPI device
</content>
</entry>
<entry>
<title>Revert "Input: Add the `REL_WHEEL_HI_RES` event code"</title>
<updated>2018-11-22T07:57:44Z</updated>
<author>
<name>Benjamin Tissoires</name>
<email>benjamin.tissoires@redhat.com</email>
</author>
<published>2018-11-21T15:27:12Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=ffe0e7cf290f5c9d1392134b4ef8da2a3761a4cd'/>
<id>urn:sha1:ffe0e7cf290f5c9d1392134b4ef8da2a3761a4cd</id>
<content type='text'>
This reverts commit aaf9978c3c0291ef3beaa97610bc9c3084656a85.

Quoting Peter:

There is a HID feature report called "Resolution Multiplier"
Described in the "Enhanced Wheel Support in Windows" doc and
the "USB HID Usage Tables" page 30.

http://download.microsoft.com/download/b/d/1/bd1f7ef4-7d72-419e-bc5c-9f79ad7bb66e/wheel.docx
https://www.usb.org/sites/default/files/documents/hut1_12v2.pdf

This was new for Windows Vista, so we're only a decade behind here. I only
accidentally found this a few days ago while debugging a stuck button on a
Microsoft mouse.

The docs above describe it like this: a wheel control by default sends
value 1 per notch. If the resolution multiplier is active, the wheel is
expected to send a value of $multiplier per notch (e.g. MS Sculpt mouse) or
just send events more often, i.e. for less physical motion (e.g. MS Comfort
mouse).

For the latter, you need the right HW of course. The Sculpt mouse has
tactile wheel clicks, so nothing really changes. The Comfort mouse has
continuous motion with no tactile clicks. Similar to the free-wheeling
Logitech mice but without any inertia.

Note that the doc also says that Vista and onwards *always* enable this
feature where available.

An example HID definition looks like this:

       Usage Page Generic Desktop (0x01)
       Usage Resolution Multiplier (0x48)
       Logical Minimum 0
       Logical Maximum 1
       Physical Minimum 1
       Physical Maximum 16
       Report Size 2 # in bits
       Report Count 1
       Feature (Data, Var, Abs)

So the actual bits have values 0 or 1 and that reflects real values 1 or 16.
We've only seen single-bits so far, so there's low-res and hi-res, but
nothing in between.

The multiplier is available for HID usages "Wheel" and "AC Pan" (horiz wheel).
Microsoft suggests that

&gt; Vendors should ship their devices with smooth scrolling disabled and allow
&gt; Windows to enable it. This ensures that the device works like a regular HID
&gt; device on legacy operating systems that do not support smooth scrolling.
(see the wheel doc linked above)

The mice that we tested so far do reset on unplug.

Device Support looks to be all (?) Microsoft mice but nothing else

Not supported:
- Logitech G500s, G303
- Roccat Kone XTD
- all the cheap Lenovo, HP, Dell, Logitech USB mice that come with a
  workstation that I could find don't have it.
- Etekcity something something
- Razer Imperator

Supported:
- Microsoft Comfort Optical Mouse 3000 - yes, physical: 1:4
- Microsoft Sculpt Ergonomic Mouse - yes, physical: 1:12
- Microsoft Surface mouse - yes, physical: 1:4

So again, I think this is really just available on Microsoft mice, but
probably all decent MS mice released over the last decade.

Looking at the hardware itself:

- no noticeable notches in the weel
- low-res: 18 events per 360deg rotation (click angle 20 deg)
- high-res: 72 events per 360deg → matches multiplier of 4

- I can feel the notches during wheel turns
- low-res: 24 events per 360 deg rotation (click angle 15 deg)
  - horiz wheel is tilt-based, continuous output value 1
- high-res: 24 events per 360deg with value 12 → matches multiplier of 12
  - horiz wheel output rate doubles/triples?, values is 3

- It's a touch strip, not a wheel so no notches
- high-res: events have value 4 instead of 1
  a bit strange given that it doesn't actually have notches.

Ok, why is this an issue for the current API? First, because the logitech
multiplier used in Harry's patches looks suspiciously like the Resolution
Multiplier so I think we should assume it's the same thing. Nestor, can you
shed some light on that?

- `REL_WHEEL` is defined as the number of notches, emulated where needed.
- `REL_WHEEL_HI_RES` is the movement of the user's finger in microns.
- `WM_MOUSEWHEEL` (Windows) is is a multiple of 120, defined as "the threshold
  for action to be taken and one such action"
  https://docs.microsoft.com/en-us/windows/desktop/inputdev/wm-mousewheel

If the multiplier is set to M, this means we need an accumulated value of M
until we can claim there was a wheel click. So after enabling the multiplier
and setting it to the maximum (like Windows):
- M units are 15deg rotation → 1 unit is 2620/M micron (see below). This is
  the `REL_WHEEL_HI_RES` value.
  - wheel diameter 20mm: 15 deg rotation is 2.62mm, 2620 micron (pi * 20mm /
    (360deg/15deg))
- For every M units accumulated, send one `REL_WHEEL` event

The problem here is that we've now hardcoded 20mm/15 deg into the kernel and
we have no way of getting the size of the wheel or the click angle into the
kernel.

In userspace we now have to undo the kernel's calculation. If our click angle
is e.g. 20 degree we have to undo the (lossy) calculation from the kernel and
calculate the correct angle instead. This also means the 15 is a hardcoded
option forever and cannot be changed.

In hid-logitech-hidpp.c, the microns per unit is hardcoded per device.
Harry, did you measure those by hand? We'd need to update the kernel for
every device and there are 10 years worth of devices from MS alone.

The multiplier default is 8 which is in the right ballpark, so I'm pretty
sure this is the same as the Resolution Multiplier, just in HID++ lingo. And
given that the 120 magic factor is what Windows uses in the end, I can't
imagine Logitech rolling their own thing here. Nestor?

And we're already fairly inaccurate with the microns anyway. The MX Anywhere
2S has a click angle of 20 degrees (18 stops) and a 17mm wheel, so a wheel
notch is approximately 2.67mm, one event at multiplier 8 (1/8 of a notch)
would be 334 micron. That's only 80% of the fallback value of 406 in the
kernel. Multiplier 6 gives us 445micron (10% off). I'm assuming multiplier 7
doesn't exist because it's not a factor of 120.

Summary:

Best option may be to simply do what Windows is doing, all the HW manufacturers
have to use that approach after all. Switch `REL_WHEEL_HI_RES` to report in
fractions of 120, with 120 being one notch and divide that by the multiplier
for the actual events. So e.g. the Logitech multiplier 8 would send value 15
for each event in hi-res mode. This can be converted in userspace to
whatever userspace needs (combined with a hwdb there that tells you wheel
size/click angle/...).

Conflicts:
	include/uapi/linux/input-event-codes.h -&gt; I kept the new
         reserved event in the code, so I had to adapt the revert
         slightly

Signed-off-by: Benjamin Tissoires &lt;benjamin.tissoires@redhat.com&gt;
Acked-by: Harry Cutts &lt;hcutts@chromium.org&gt;
Acked-by: Dmitry Torokhov &lt;dmitry.torokhov@gmail.com&gt;
Acked-by: Jiri Kosina &lt;jkosina@suse.cz&gt;
</content>
</entry>
<entry>
<title>Revert "HID: input: Create a utility class for counting scroll events"</title>
<updated>2018-11-22T07:57:38Z</updated>
<author>
<name>Benjamin Tissoires</name>
<email>benjamin.tissoires@redhat.com</email>
</author>
<published>2018-11-21T15:27:11Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=f1539a0c2545d7bd82e451bd1464f2a820f55de4'/>
<id>urn:sha1:f1539a0c2545d7bd82e451bd1464f2a820f55de4</id>
<content type='text'>
This reverts commit 1ff2e1a44e02d4bdbb9be67c7d9acc240a67141f.

It turns out the current API is not that compatible with
some Microsoft mice, so better start again from scratch.

Signed-off-by: Benjamin Tissoires &lt;benjamin.tissoires@redhat.com&gt;
Acked-by: Harry Cutts &lt;hcutts@chromium.org&gt;
Acked-by: Dmitry Torokhov &lt;dmitry.torokhov@gmail.com&gt;
Acked-by: Jiri Kosina &lt;jkosina@suse.cz&gt;
</content>
</entry>
<entry>
<title>tcp: defer SACK compression after DupThresh</title>
<updated>2018-11-21T23:49:52Z</updated>
<author>
<name>Eric Dumazet</name>
<email>edumazet@google.com</email>
</author>
<published>2018-11-20T13:53:59Z</published>
<link rel='alternate' type='text/html' href='https://sre.ring0.de/linux/commit/?id=86de5921a3d5dd246df661e09bdd0a6131b39ae3'/>
<id>urn:sha1:86de5921a3d5dd246df661e09bdd0a6131b39ae3</id>
<content type='text'>
Jean-Louis reported a TCP regression and bisected to recent SACK
compression.

After a loss episode (receiver not able to keep up and dropping
packets because its backlog is full), linux TCP stack is sending
a single SACK (DUPACK).

Sender waits a full RTO timer before recovering losses.

While RFC 6675 says in section 5, "Algorithm Details",

   (2) If DupAcks &lt; DupThresh but IsLost (HighACK + 1) returns true --
       indicating at least three segments have arrived above the current
       cumulative acknowledgment point, which is taken to indicate loss
       -- go to step (4).
...
   (4) Invoke fast retransmit and enter loss recovery as follows:

there are old TCP stacks not implementing this strategy, and
still counting the dupacks before starting fast retransmit.

While these stacks probably perform poorly when receivers implement
LRO/GRO, we should be a little more gentle to them.

This patch makes sure we do not enable SACK compression unless
3 dupacks have been sent since last rcv_nxt update.

Ideally we should even rearm the timer to send one or two
more DUPACK if no more packets are coming, but that will
be work aiming for linux-4.21.

Many thanks to Jean-Louis for bisecting the issue, providing
packet captures and testing this patch.

Fixes: 5d9f4262b7ea ("tcp: add SACK compression")
Reported-by: Jean-Louis Dupond &lt;jean-louis@dupond.be&gt;
Tested-by: Jean-Louis Dupond &lt;jean-louis@dupond.be&gt;
Signed-off-by: Eric Dumazet &lt;edumazet@google.com&gt;
Acked-by: Neal Cardwell &lt;ncardwell@google.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
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