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Flash partitions doc#2

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Flash partitions doc#2
nvlsianpu wants to merge 362 commits intomasterfrom
flash_partitions_doc

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pfalcon and others added 30 commits December 29, 2017 09:45
Of these, only struct net_ipv6_nbr_data::send_ns is a descriptive
change:

send_ns is used for timing Neighbor Solicitations in general, not
just for DAD.

The rest are typo/grammar fixes.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Fix the parsing of IVIndex value from shell command line for
cmd_provision

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Using the '\$' technique to accept make variables corrupts Ninja build
systems, so avoid using it when generating Ninja files. Not using it
with Ninja means we need to come up with some other mechanism to
support build-time configuration of the QEMU CLI with Ninja.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
fin_timer will be started after sending FIN to peer. After successful
reception of FIN+ACK message in FIN_WAIT1 state, fin_timer should be
cancelled.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>
Right now in FIN_WAIT1 state, if we receive FIN+ACK message, then
tcp state changed to FIN_WAIT2 on ACK flag and immediately on FIN
flag state changed to TIME_WAIT. Then final ACK is prepared and sent
(in queue at-least) to peer. Again immediately state changed to
TCP_CLOSED, where context is freed. net_context_put frees context
and releases tcp connection. Final ACK packet which is in queue
is dropped.

As a side effect of freed ACK packet, peer device keep on sending
FIN+ACK messages (that's why we see a lot of "TCP spurious
retransimission" messages in wireshark). As a result
of context free (respective connection handler also removed), we see
lot of packets dropped at connection input handler and replying with
ICMP error messages (destination unreachable).

To fix this issue, timewait timer support is required. When tcp
connection state changed to TIMEWAIT state, it should wait until
TIMEWAIT_TIMETOUT before changing state to TCP_CLOSED. It's
appropriate to close the tcp connection after timewait timer expiry.

Note: Right now timeout value is constant (250ms). But it should
be 2 * MSL (Maximum segment lifetime).

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>
Do not reset net_context information in net_app_close. In case of
TCP behind the curtains connection might wait for some timers to
expire and send some messages (e.g. ACK). If we set source port to
'0', unexpected behaviour might happen with peer connection.
Only reset net_app context related information on net_app_close.
Let net_context_put will take care of proper connection closure.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>
The gap between 1st and 2nd column is only one character.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
This updates GAP, GATT, SM, L2CAP ICS values according to following
documents:
GAP.ICS.5.0.2
SM.ICS.5.0.0
GATT.ICS.5.0.1
L2CAP.ICS.5.0.1

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
We now have coap, all dependencies have been removed, so cleanup for the
next release.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
I don't know why we have been passing "-serial none" to qemu but it
does not seem to be doing anything useful. So I propose we remove it.

According to the qemu documenation it should be disabling all serial
ports;

"-serial dev
Redirect the virtual serial port to host character device dev. The
default device is vc in graphical mode and stdio in non graphical
mode.

This option can be used several times to simulate up to 4 serial
ports.

Use -serial none to disable all serial ports."

But when we use "-serial none", we always combine it with "-serial
pipe", or "-serial unix", to redirect the serial port, so clearly it
is not disabling all serial ports as it attempting to do.

"-serial none" was first introduced to Zephyr in this commit:
0b54f98

As far I can tell, pinging an echo_server works fine without it, so I
suspect that our second -serial flag is overriding it and that it has
no effect.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
This board doesn't support `make flash` way of programming.
So, remove that from documentation.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
We have been passing around options from one function to the next making
it very difficult to add a new option easily and requiring changes to
man function prototypes.
This declated the parsed command line options global and renames args to
options. args is being used elsewhere and this was confusing.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Now use glabal options instead of passing them around as function
arguments.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Add an option to support building with ninja instead of make.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
This map is not being used anymore.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
We were encoding logs twice.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Use codecov.io to track test coverage.

See https://codecov.io/gh/zephyrproject-rtos/zephyr

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
ETH_MCUX_0 needs to be turned on when CONFIG_NET_L2_ETHERNET is enabled.
If CONFIG_NET_L2_ETHERNET is disabled (e.g. when configured for 6loble),
then ETH_MCUX_0 should remain disabled.

Signed-off-by: Vakul Garg <vakul.garg@nxp.com>
Corrected types in OS specific functions to be inline with NFFS
declarations.

Fixes zephyrproject-rtos#5467

Signed-off-by: Jan Van Winkel <jan.van_winkel@dxplore.eu>
When SIGTERM or SIGINT are received handle them
by gracefully exiting the program.

Fixes zephyrproject-rtos#5477

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>
Introduce Zephyr interface libraries to fix zephyrproject-rtos#5351.

From the documentation:

A Zephyr interface library is a thin wrapper over a CMake INTERFACE
library. The most important responsibility of this abstraction is to
ensure that when a user KConfig-enables a library then the header
files of this library will be accessible to the 'app' library.

This is done because when a user uses Kconfig to enable a library he
expects to be able to include it's header files and call it's
functions out-of-the box.

A Zephyr interface library should be used when there exists some
build information (include directories, defines, compiler flags,
etc.) that should be applied to a set of Zephyr libraries and 'app'
might be one of these libraries.

Zephyr libraries must explicitly call
zephyr_library_link_libraries(<interface_library>) to use this build
information. 'app' is treated as a special case for usability
reasons; a Kconfig option (CONFIG_APP_LINK_WITH_<interface_library>)
should exist for each interface_library and will determine if 'app'
links with the interface_library.

This API has a constructor like the zephyr_library API has, but it

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
mbedTLS include directories will now default to be in the 'app'
include path when mbedTLS has been enabled.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
This command will be used to test if model can properly send
segmented and unsegmented messages to a given destination address.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
This may fail, if model publication was not set by remote.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
This adds BTP event indicating reception of provisioning message
with invalid (RFU) BearerOpcode.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
This adds commands to manage Friend node Subscription List.
Those will be used to add or remove and group/virtual address
from subscription list.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
This command is needed for Bluetooth Mesh testing against PTS.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
This function is called only within mesh stack.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
This event indicates that segmented message incomplete timer
expired.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
Signed-off-by: Dong Xiang <dong.xiang@linaro.org>
nvlsianpu and others added 14 commits January 15, 2018 15:46
Tests added were adopted from MyNewt.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Moved fetch of flash device bindings to early initialization of the
application.
Device bindings are constant while the application is running so
it is better to fetch it at startup, and not every time flash_map
procedures are called.


Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Document USB Vendor and Product IDs and their intended usage.
Set the Vendor and Product IDs and define the USB bcdDevice
Device Descriptor Device Release Number to be the binary
coded decimal representation of the Zephyr major and minor
kernel version number.

Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>
Due to a chicken-and-egg problem, it's not possible to call
board_runner_args() directly from an application CMakeLists.txt and
have it take effect:

  - if the board_runner_args() call appears before including the
    boilerplate file, then the function is undefined and an error
    occurs.

  - if the call appears after including the boilerplate file, then
    board_finalize_runner_args() has already been called by the
    relevant runner.board.cmake file, so it won't take effect.

Similarly to the way set_conf_file() works, allow applications to
declare additional runner args by checking if they have provided a
macro named app_set_runner_args(), which contains calls to
board_runner_args(). Using a macro means that we can invoke it with
the definition of board_runner_args() in scope.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
Remove a few duplicated entries and update maintainers for some code.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Update tags.sh tool to generate appropriate files to use with cscope and
ctags programs.

Current implementation generates tags for all but samples and it's
possible to filter by architecture.

Signed-off-by: Juan Manuel Torres Palma <j.m.torrespalma@gmail.com>
The jquery provided by Sphinx 1.5.5 has a bug dealing with the
coordinates returned for offset().top for empty <span> tags, generated
by Sphinx for intra-document labels used to create hyperlink anchor
targets.  As reported in PR zephyrproject-rtos#5591, some browsers (chrome for example)
don't scroll to the right spot in the document when we link to these
intra-document label points.  jquery v3.1.2 fixes this.  (When Sphinx
updates the jquery included in its release, we can remove our copy that
is overriding the copy installed in the generated output by Sphinx.)

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>
BT does use a semaphore, which does cause the count of sempahores to
fail, disable BT here to only keep locally created objects.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
When checking if a testcase passed or failed, allow there
to be prefixes or postfixes in the line, around
PROJECT EXECUTION SUCCESSFUL/FAILED

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>
This test should be blacklisted for any board based
on the POSIX arch, not just the native_posix board

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>
Make it possible to run in other posix boards.
By default, if the POSIX board does not define the TICK_IRQ
just run without that part of the test, printing a note.
The place where other POSIX boards should define it, is also
clear, and should be easy to keep those lines free from merge
conflicts in the future.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>
The offset of the IP header in a received packet depends on the L2
header size.  For Ethernet this is 14 bytes which puts the u32 IPv4
addresses on a non-u32 byte boundary.  This causes chips that don't
support unaligned access (like the Cortex-M0) to fault.

The fixes in this patch are enough to ping the board and run the
http_server sample.

Signed-off-by: Michael Hope <mlhx@google.com>
This removes BT_DEBUG dependency on BT_TESTING flag.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
This fixes build error due to undeclared CONFIG_UART_PIPE_ON_DEV_NAME
on nrf52840 board.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
@nvlsianpu nvlsianpu force-pushed the flash_partitions_doc branch from c14fbfd to f17a704 Compare January 25, 2018 07:08
This documentation is needed for developers who are porting new boards.
Fixing zephyrproject-rtos#5577


Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
@nvlsianpu nvlsianpu force-pushed the flash_partitions_doc branch from f17a704 to 95ed395 Compare January 25, 2018 07:18
@nvlsianpu nvlsianpu closed this Jan 25, 2018
nvlsianpu pushed a commit that referenced this pull request Mar 21, 2018
The scheduler exposed two APIs to do the same thing:
_add_thread_to_ready_q() was a low level primitive that in most cases
was wrapped by _ready_thread(), which also (1) checks that the thread
_is_ready() or exits, (2) flags the thread as "started" to handle the
case of a thread running for the first time out of a waitq timeout,
and (3) signals a logger event.

As it turns out, all existing usage was already checking case #1.
Case #2 can be better handled in the timeout resume path instead of on
every call.  And case #3 was probably wrong to have been skipping
anyway (there were paths that could make a thread runnable without
logging).

Now _add_thread_to_ready_q() is an internal scheduler API, as it
probably always should have been.

This also moves some asserts from the inline _ready_thread() wrapper
to the underlying true function for code size reasons, otherwise the
extra use of the inline added by this patch blows past code size
limits on Quark D2000.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
nvlsianpu pushed a commit that referenced this pull request Jul 4, 2019
Currently, the free block bitmap is roughly 4 times larger than it
needs to, wasting memory.

Let's assume maxsz = 128, minsz = 8 and n_max = 40.

Z_MPOOL_LVLS(128, 8) returns 3. The block size for level #0 is 128,
the block size for level #1 is 128/4 = 32, and the block size for
level #2 is 32/4 = 8. Hence levels 0, 1, and 2 for a total of 3 levels.
So far so good.

Now let's look at Z_MPOOL_LBIT_WORDS(). We get:

Z_MPOOL_LBIT_WORDS_UNCLAMPED(40, 0) = ((40 << 0) + 31) / 32 = 2
Z_MPOOL_LBIT_WORDS_UNCLAMPED(40, 1) = ((40 << 2) + 31) / 32 = 5
Z_MPOOL_LBIT_WORDS_UNCLAMPED(40, 2) = ((40 << 4) + 31) / 32 = 20

None of those are < 2 so Z_MPOOL_LBIT_WORDS() takes the results from
Z_MPOOL_LBIT_WORDS_UNCLAMPED().

Finally, let's look at _MPOOL_BITS_SIZE(. It sums all possible levels
with Z_MPOOL_LBIT_BYTES() which is:

  #define Z_MPOOL_LBIT_BYTES(maxsz, minsz, l, n_max)    \
        (Z_MPOOL_LVLS((maxsz), (minsz)) >= (l) ?        \
         4 * Z_MPOOL_LBIT_WORDS((n_max), l) : 0)

Or given what we already have:

Z_MPOOL_LBIT_BYTES(128, 8, 0, 40) = (3 >= 0) ? 4 * 2  : 0 = 8
Z_MPOOL_LBIT_BYTES(128, 8, 1, 40) = (3 >= 1) ? 4 * 5  : 0 = 20
Z_MPOOL_LBIT_BYTES(128, 8, 2, 40) = (3 >= 2) ? 4 * 20 : 0 = 80
Z_MPOOL_LBIT_BYTES(128, 8, 3, 40) = (3 >= 3) ? 4 * ??

Wait... we're missing this one:

Z_MPOOL_LBIT_WORDS_UNCLAMPED(40, 3) = ((40 << 6) + 31) / 32 = 80

then:

Z_MPOOL_LBIT_BYTES(128, 8, 3, 40) = (3 >= 3) ? 4 * 80 : 0 = 320

Further levels yeld (3 >= 4), (3 >= 5), etc. so they're all false and
produce 0.

So this means that we're statically allocating 428 bytes to the bitmap
when clearly only the first 3 Z_MPOOL_LBIT_BYTES() results for the
corresponding 3 levels that we have should be summed e.g. only
108 bytes.

Here the code logic gets confused between level numbers and the number
levels, hence the extra allocation which happens to be exponential.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
nvlsianpu pushed a commit that referenced this pull request Oct 10, 2019
Fix Kconfig conditional include of Minimum Channels Used
and Channel Selection Algorithm #2.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
nvlsianpu pushed a commit that referenced this pull request Jun 23, 2020
This makes the gatt metrics also available for
gatt write-without-rsp-cb so it now prints the rate of each write:

uart:~$ gatt write-without-response-cb 1e ff 10 10
Write #1: 16 bytes (0 bps)
Write #2: 32 bytes (3445948416 bps)
Write #3: 48 bytes (2596929536 bps)
Write zephyrproject-rtos#4: 64 bytes (6400 bps)
Write zephyrproject-rtos#5: 80 bytes (8533 bps)
Write zephyrproject-rtos#6: 96 bytes (10666 bps)
Write zephyrproject-rtos#7: 112 bytes (8533 bps)
Write zephyrproject-rtos#8: 128 bytes (9955 bps)
Write zephyrproject-rtos#9: 144 bytes (11377 bps)
Write zephyrproject-rtos#10: 160 bytes (7680 bps)
Write zephyrproject-rtos#11: 176 bytes (8533 bps)
Write zephyrproject-rtos#12: 192 bytes (9386 bps)
Write Complete (err 0)
Write zephyrproject-rtos#13: 208 bytes (8533 bps)
Write zephyrproject-rtos#14: 224 bytes (9244 bps)
Write zephyrproject-rtos#15: 240 bytes (9955 bps)
Write zephyrproject-rtos#16: 256 bytes (8000 bps)

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
nvlsianpu pushed a commit that referenced this pull request Nov 10, 2020
The _ldiv5() is an optimized divide-by-5 function that is smaller and
faster than the generic libgcc implementation.

Yet it can be made even smaller and faster with this replacement
implementation based on a reciprocal multiplication plus some tricks.

For example, here's the assembly from the original code on ARM:

_ldiv5:
        ldr     r3, [r0]
        movw    ip, zephyrproject-rtos#52429
        ldr     r1, [r0, zephyrproject-rtos#4]
        movt    ip, 52428
        adds    r3, r3, #2
        push    {r4, r5, r6, r7, lr}
        mov     lr, #0
        adc     r1, r1, lr
        adds    r2, lr, lr
        umull   r7, r6, ip, r1
        lsr     r6, r6, #2
        adc     r7, r6, r6
        adds    r2, r2, r2
        adc     r7, r7, r7
        adds    r2, r2, lr
        adc     r7, r7, r6
        subs    r3, r3, r2
        sbc     r7, r1, r7
        lsr     r2, r3, #3
        orr     r2, r2, r7, lsl zephyrproject-rtos#29
        umull   r2, r1, ip, r2
        lsr     r2, r1, #2
        lsr     r7, r1, zephyrproject-rtos#31
        lsl     r1, r2, #3
        adds    r4, lr, r1
        adc     r5, r6, r7
        adds    r2, r1, r1
        adds    r2, r2, r2
        adds    r2, r2, r1
        subs    r2, r3, r2
        umull   r3, r2, ip, r2
        lsr     r2, r2, #2
        adds    r4, r4, r2
        adc     r5, r5, #0
        strd    r4, [r0]
        pop     {r4, r5, r6, r7, pc}

And here's the resulting assembly with this commit applied:

_ldiv5:
        push    {r4, r5, r6, r7}
        movw    r4, zephyrproject-rtos#13107
        ldr     r6, [r0]
        movt    r4, 13107
        ldr     r1, [r0, zephyrproject-rtos#4]
        mov     r3, #0
        umull   r6, r7, r6, r4
        add     r2, r4, r4, lsl #1
        umull   r4, r5, r1, r4
        adds    r1, r6, r2
        adc     r2, r7, r2
        adds    ip, r6, r4
        adc     r1, r7, r5
        adds    r2, ip, r2
        adc     r2, r1, r3
        adds    r2, r4, r2
        adc     r3, r5, r3
        strd    r2, [r0]
        pop     {r4, r5, r6, r7}
        bx      lr

So we're down to 20 instructions from 36 initially, with only 2 umull
instructions instead of 3, and slightly smaller stack footprint.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
nvlsianpu pushed a commit that referenced this pull request Mar 22, 2021
Added function to calculate channel identifier value
required for Channel Selection Algorithm #2.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
nvlsianpu pushed a commit that referenced this pull request Mar 26, 2021
The fatal log now contains
- Trap type in human readable representation
- Integer registers visible to the program when trap was taken
- Special register values such as PC and PSR
- Backtrace with PC and SP

If CONFIG_EXTRA_EXCEPTION_INFO is enabled, then all the above is
logged. If not, only the special registers are logged.

The format is inspired by the GRMON debug monitor and TSIM simulator.
A quick guide on how to use the values is in fatal.c.

It now looks like this:

E: tt = 0x02, illegal_instruction
E:
E:       INS        LOCALS     OUTS       GLOBALS
E:   0:  00000000   f3900fc0   40007c50   00000000
E:   1:  00000000   40004bf0   40008d30   40008c00
E:   2:  00000000   40004bf4   40008000   00000003
E:   3:  40009158   00000000   40009000   00000002
E:   4:  40008fa8   40003c00   40008fa8   00000008
E:   5:  40009000   f3400fc0   00000000   00000080
E:   6:  4000a1f8   40000050   4000a190   00000000
E:   7:  40002308   00000000   40001fb8   000000c1
E:
E: psr: f30000c7   wim: 00000008   tbr: 40000020   y: 00000000
E:  pc: 4000a1f4   npc: 4000a1f8
E:
E:       pc         sp
E:  #0   4000a1f4   4000a190
E:  #1   40002308   4000a1f8
E:  #2   40003b24   4000a258

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>
nvlsianpu pushed a commit that referenced this pull request May 6, 2021
Reorder Channel Selection Algorithm #2 and Minimum Number of
Used Channels Procedure bits.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
nvlsianpu pushed a commit that referenced this pull request Jul 26, 2021
Implement the functions required to calculate the SubEvent 1
and SubEvent n mapped channel indices.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
nvlsianpu pushed a commit that referenced this pull request Jul 26, 2021
Added Bluetooth Specification references to the
implementation of Channel Selection algorithm #2 in the
Controller.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
nvlsianpu pushed a commit that referenced this pull request Oct 28, 2021
Remove explicit disable of Channel Selection Algorithm #2
in the mesh tests that use Extended Advertising.

Fixes zephyrproject-rtos#39188.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
nvlsianpu pushed a commit that referenced this pull request Dec 8, 2022
This patch reworks how fragments are handled in the net_buf
infrastructure.

In particular, it removes the union around the node and frags members in
the main net_buf structure. This is done so that both can be used at the
same time, at a cost of 4 bytes per net_buf instance.
This implies that the layout of net_buf instances changes whenever being
inserted into a queue (fifo or lifo) or a linked list (slist).

Until now, this is what happened when enqueueing a net_buf with frags in
a queue or linked list:

1.1 Before enqueueing:

 +--------+      +--------+      +--------+
 |#1  node|\     |#2  node|\     |#3  node|\
 |        | \    |        | \    |        | \
 | frags  |------| frags  |------| frags  |------NULL
 +--------+      +--------+      +--------+

net_buf #1 has 2 fragments, net_bufs #2 and #3. Both the node and frags
pointers (they are the same, since they are unioned) point to the next
fragment.

1.2 After enqueueing:

 +--------+      +--------+      +--------+      +--------+      +--------+
 |q/slist |------|#1  node|------|#2  node|------|#3  node|------|q/slist |
 |node    |      | *flag  | /    | *flag  | /    |        | /    |node    |
 |        |      | frags  |/     | frags  |/     | frags  |/     |        |
 +--------+      +--------+      +--------+      +--------+      +--------+

When enqueing a net_buf (in this case #1) that contains fragments, the
current net_buf implementation actually enqueues all the fragments (in
this case #2 and #3) as actual queue/slist items, since node and frags
are one and the same in memory. This makes the enqueuing operation
expensive and it makes it impossible to atomically dequeue. The `*flag`
notation here means that the `flags` member has been set to
`NET_BUF_FRAGS` in order to be able to reconstruct the frags pointers
when dequeuing.

After this patch, the layout changes considerably:

2.1 Before enqueueing:

 +--------+       +--------+       +--------+
 |#1  node|--NULL |#2  node|--NULL |#3  node|--NULL
 |        |       |        |       |        |
 | frags  |-------| frags  |-------| frags  |------NULL
 +--------+       +--------+       +--------+

This is very similar to 1.1, except that now node and frags are
different pointers, so node is just set to NULL.

2.2 After enqueueing:

 +--------+       +--------+       +--------+
 |q/slist |-------|#1  node|-------|q/slist |
 |node    |       |        |       |node    |
 |        |       | frags  |       |        |
 +--------+       +--------+       +--------+
                      |            +--------+       +--------+
                      |            |#2  node|--NULL |#3  node|--NULL
                      |            |        |       |        |
                      +------------| frags  |-------| frags  |------NULL
                                   +--------+       +--------+

When enqueuing net_buf #1, now we only enqueue that very item, instead
of enqueing the frags as well, since now node and frags are separate
pointers. This simplifies the operation and makes it atomic.

Resolves zephyrproject-rtos#52718.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
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