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Vladimir Oltean e7d48e5fbf net: enetc: add a mini driver for the Integrated Endpoint Register Block
The NXP ENETC is a 4-port Ethernet controller which 'smells' to
operating systems like 4 distinct PCIe PFs with SR-IOV, each PF having
its own driver instance, but in fact there are some hardware resources
which are shared between all ports, like for example the 256 KB SRAM
FIFO between the MACs and the Host Transfer Agent which DMAs frames to
DRAM.

To hide the stuff that cannot be neatly exposed per port, the hardware
designers came up with this idea of having a dedicated register block
which is supposed to be populated by the bootloader, and contains
everything configuration-related: MAC addresses, FIFO partitioning, etc.

When a port is reset using PCIe Function Level Reset, its defaults are
transferred from the IERB configuration. Most of the time, the settings
made through the IERB are read-only in the port's memory space (if they
are even visible), so they cannot be modified at runtime.

Linux doesn't have any advanced FIFO partitioning requirements at all,
but when reading through the hardware manual, it became clear that, even
though there are many good 'recommendations' for default values, many of
them were not actually put in practice on LS1028A. So we end up with a
default configuration that:

(a) does not have enough TX and RX byte credits to support the max MTU
    of 9600 (which the Linux driver claims already) properly (at full speed)
(b) allows the FIFO to be overrun with RX traffic, potentially
    overwriting internal data structures.

The last part sounds a bit catastrophic, but it isn't. Frames are
supposed to transit the FIFO for a very short time, but they can
actually accumulate there under 2 conditions:

(a) there is very severe congestion on DRAM memory, or
(b) the RX rings visible to the operating system were configured for
    lossless operation, and they just ran out of free buffers to copy
    the frame to. This is what is used to put backpressure onto the MAC
    with flow control.

So since ENETC has not supported flow control thus far, RX FIFO overruns
were never seen with Linux. But with the addition of flow control, we
should configure some registers to prevent this from happening. What we
are trying to protect against are bad actors which continue to send us
traffic despite the fact that we have signaled a PAUSE condition. Of
course we can't be lossless in that case, but it is best to configure
the FIFO to do tail dropping rather than letting it overrun.

So in a nutshell, this driver is a fixup for all the IERB default values
that should have been but aren't.

The IERB configuration needs to be done _before_ the PFs are enabled.
So every PF searches for the presence of the "fsl,ls1028a-enetc-ierb"
node in the device tree, and if it finds it, it "registers" with the
IERB, which means that it requests the IERB to fix up its default
values. This is done through -EPROBE_DEFER. The IERB driver is part of
the fsl_enetc module, but is technically a platform driver, since the
IERB is a good old fashioned MMIO region, as opposed to ENETC ports
which pretend to be PCIe devices.

The driver was already configuring ENETC_PTXMBAR (FIFO allocation for
TX) because due to an omission, TXMBAR is a read/write register in the
PF memory space. But the manual is quite clear that the formula for this
should depend upon the TX byte credits (TXBCR). In turn, the TX byte
credits are only readable/writable through the IERB. So if we want to
ensure that the TXBCR register also has a value that is correct and in
line with TXMBAR, there is simply no way this can be done from the PF
driver, access to the IERB is needed.

I could have modified U-Boot to fix up the IERB values, but that is
quite undesirable, as old U-Boot versions are likely to be floating
around for quite some time from now.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
2021-04-19 15:31:45 -07:00
arch Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-04-17 11:08:07 -07:00
block block: remove the unused RQF_ALLOCED flag 2021-04-02 11:18:31 -06:00
certs certs: Replace K{U,G}IDT_INIT() with GLOBAL_ROOT_{U,G}ID 2021-01-21 16:16:10 +00:00
crypto crypto: mips/poly1305 - enable for all MIPS processors 2021-03-08 11:52:17 +01:00
Documentation dt-bindings: net: fsl: enetc: add the IERB documentation 2021-04-19 15:31:45 -07:00
drivers net: enetc: add a mini driver for the Integrated Endpoint Register Block 2021-04-19 15:31:45 -07:00
fs Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-04-17 11:08:07 -07:00
include Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-04-17 11:08:07 -07:00
init Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-03-25 15:31:22 -07:00
ipc fs: make helpers idmap mount aware 2021-01-24 14:27:20 +01:00
kernel Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-04-17 11:08:07 -07:00
lib Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-04-17 11:08:07 -07:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
mm mm: ptdump: fix build failure 2021-04-16 16:10:37 -07:00
net ethtool: ioctl: Fix out-of-bounds warning in store_link_ksettings_for_user() 2021-04-19 15:27:37 -07:00
samples samples: bpf: Do not unload prog within xdpsock 2021-03-30 09:24:39 -07:00
scripts Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-04-17 11:08:07 -07:00
security Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-04-17 11:08:07 -07:00
sound sound fixes for 5.12-rc7 2021-04-08 09:01:30 -07:00
tools selftests: fib_nexthops: Test large scale nexthop flushing 2021-04-19 15:20:34 -07:00
usr Kbuild updates for v5.12 2021-02-25 10:17:31 -08:00
virt KVM: x86/mmu: Consider the hva in mmu_notifier retry 2021-02-22 13:16:53 -05:00
.clang-format cxl for 5.12 2021-02-24 09:38:36 -08:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore clang-lto series for v5.12-rc1 2021-02-23 09:28:51 -08:00
.mailmap .mailmap: fix old email addresses 2021-04-09 14:54:23 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS net: mana: Add a driver for Microsoft Azure Network Adapter (MANA) 2021-04-19 15:24:25 -07:00
Makefile Linux 5.12-rc7 2021-04-11 15:16:13 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.