src/kernel/linux/v4.14/Documentation/networking/stmmac.txt

       STMicroelectronics 10/100/1000 Synopsys Ethernet driver

Copyright (C) 2007-2015  STMicroelectronics Ltd
Author: Giuseppe Cavallaro <peppe.cavallaro@st.com>

This is the driver for the MAC 10/100/1000 on-chip Ethernet controllers
(Synopsys IP blocks).

Currently this network device driver is for all STi embedded MAC/GMAC
(i.e. 7xxx/5xxx SoCs), SPEAr (arm), Loongson1B (mips) and XLINX XC2V3000
FF1152AMT0221 D1215994A VIRTEX FPGA board.

DWC Ether MAC 10/100/1000 Universal version 3.70a (and older) and DWC Ether
MAC 10/100 Universal version 4.0 have been used for developing this driver.

This driver supports both the platform bus and PCI.

Please, for more information also visit: www.stlinux.com

1) Kernel Configuration
The kernel configuration option is STMMAC_ETH:
 Device Drivers ---> Network device support ---> Ethernet (1000 Mbit) --->
 STMicroelectronics 10/100/1000 Ethernet driver (STMMAC_ETH)

CONFIG_STMMAC_PLATFORM: is to enable the platform driver.
CONFIG_STMMAC_PCI: is to enable the pci driver.

2) Driver parameters list:
	debug: message level (0: no output, 16: all);
	phyaddr: to manually provide the physical address to the PHY device;
	buf_sz: DMA buffer size;
	tc: control the HW FIFO threshold;
	watchdog: transmit timeout (in milliseconds);
	flow_ctrl: Flow control ability [on/off];
	pause: Flow Control Pause Time;
	eee_timer: tx EEE timer;
	chain_mode: select chain mode instead of ring.

3) Command line options
Driver parameters can be also passed in command line by using:
	stmmaceth=watchdog:100,chain_mode=1

4) Driver information and notes

4.1) Transmit process
The xmit method is invoked when the kernel needs to transmit a packet; it sets
the descriptors in the ring and informs the DMA engine, that there is a packet
ready to be transmitted.
By default, the driver sets the NETIF_F_SG bit in the features field of the
net_device structure, enabling the scatter-gather feature. This is true on
chips and configurations where the checksum can be done in hardware.
Once the controller has finished transmitting the packet, timer will be
scheduled to release the transmit resources.

4.2) Receive process
When one or more packets are received, an interrupt happens. The interrupts
are not queued, so the driver has to scan all the descriptors in the ring during
the receive process.
This is based on NAPI, so the interrupt handler signals only if there is work
to be done, and it exits.
Then the poll method will be scheduled at some future point.
The incoming packets are stored, by the DMA, in a list of pre-allocated socket
buffers in order to avoid the memcpy (zero-copy).

4.3) Interrupt mitigation
The driver is able to mitigate the number of its DMA interrupts
using NAPI for the reception on chips older than the 3.50.
New chips have an HW RX-Watchdog used for this mitigation.
Mitigation parameters can be tuned by ethtool.

4.4) WOL
Wake up on Lan feature through Magic and Unicast frames are supported for the
GMAC core.

4.5) DMA descriptors
Driver handles both normal and alternate descriptors. The latter has been only
tested on DWC Ether MAC 10/100/1000 Universal version 3.41a and later.

STMMAC supports DMA descriptor to operate both in dual buffer (RING)
and linked-list(CHAINED) mode. In RING each descriptor points to two
data buffer pointers whereas in CHAINED mode they point to only one data
buffer pointer. RING mode is the default.

In CHAINED mode each descriptor will have pointer to next descriptor in
the list, hence creating the explicit chaining in the descriptor itself,
whereas such explicit chaining is not possible in RING mode.

4.5.1) Extended descriptors
The extended descriptors give us information about the Ethernet payload
when it is carrying PTP packets or TCP/UDP/ICMP over IP.
These are not available on GMAC Synopsys chips older than the 3.50.
At probe time the driver will decide if these can be actually used.
This support also is mandatory for PTPv2 because the extra descriptors
are used for saving the hardware timestamps and Extended Status.

4.6) Ethtool support
Ethtool is supported.

For example, driver statistics (including RMON), internal errors can be taken
using:
  # ethtool -S ethX
command

4.7) Jumbo and Segmentation Offloading
Jumbo frames are supported and tested for the GMAC.
The GSO has been also added but it's performed in software.
LRO is not supported.

4.8) Physical
The driver is compatible with Physical Abstraction Layer to be connected with
PHY and GPHY devices.

4.9) Platform information
Several information can be passed through the platform and device-tree.

struct plat_stmmacenet_data {
	char *phy_bus_name;
	int bus_id;
	int phy_addr;
	int interface;
	struct stmmac_mdio_bus_data *mdio_bus_data;
	struct stmmac_dma_cfg *dma_cfg;
	int clk_csr;
	int has_gmac;
	int enh_desc;
	int tx_coe;
	int rx_coe;
	int bugged_jumbo;
	int pmt;
	int force_sf_dma_mode;
	int force_thresh_dma_mode;
	int riwt_off;
	int max_speed;
	int maxmtu;
	void (*fix_mac_speed)(void *priv, unsigned int speed);
	void (*bus_setup)(void __iomem *ioaddr);
	int (*init)(struct platform_device *pdev, void *priv);
	void (*exit)(struct platform_device *pdev, void *priv);
	void *bsp_priv;
	int has_gmac4;
	bool tso_en;
};

Where:
 o phy_bus_name: phy bus name to attach to the stmmac.
 o bus_id: bus identifier.
 o phy_addr: the physical address can be passed from the platform.
	    If it is set to -1 the driver will automatically
	    detect it at run-time by probing all the 32 addresses.
 o interface: PHY device's interface.
 o mdio_bus_data: specific platform fields for the MDIO bus.
 o dma_cfg: internal DMA parameters
   o pbl: the Programmable Burst Length is maximum number of beats to
       be transferred in one DMA transaction.
       GMAC also enables the 4xPBL by default. (8xPBL for GMAC 3.50 and newer)
   o txpbl/rxpbl: GMAC and newer supports independent DMA pbl for tx/rx.
   o pblx8: Enable 8xPBL (4xPBL for core rev < 3.50). Enabled by default.
   o fixed_burst/mixed_burst/aal
 o clk_csr: fixed CSR Clock range selection.
 o has_gmac: uses the GMAC core.
 o enh_desc: if sets the MAC will use the enhanced descriptor structure.
 o tx_coe: core is able to perform the tx csum in HW.
 o rx_coe: the supports three check sum offloading engine types:
	   type_1, type_2 (full csum) and no RX coe.
 o bugged_jumbo: some HWs are not able to perform the csum in HW for
		over-sized frames due to limited buffer sizes.
		Setting this flag the csum will be done in SW on
		JUMBO frames.
 o pmt: core has the embedded power module (optional).
 o force_sf_dma_mode: force DMA to use the Store and Forward mode
		     instead of the Threshold.
 o force_thresh_dma_mode: force DMA to use the Threshold mode other than
		     the Store and Forward mode.
 o riwt_off: force to disable the RX watchdog feature and switch to NAPI mode.
 o fix_mac_speed: this callback is used for modifying some syscfg registers
		 (on ST SoCs) according to the link speed negotiated by the
		 physical layer .
 o bus_setup: perform HW setup of the bus. For example, on some ST platforms
	     this field is used to configure the AMBA  bridge to generate more
	     efficient STBus traffic.
 o init/exit: callbacks used for calling a custom initialization;
	     this is sometime necessary on some platforms (e.g. ST boxes)
	     where the HW needs to have set some PIO lines or system cfg
	     registers.  init/exit callbacks should not use or modify
	     platform data.
 o bsp_priv: another private pointer.
 o has_gmac4: uses GMAC4 core.
 o tso_en: Enables TSO (TCP Segmentation Offload) feature.

For MDIO bus The we have:

 struct stmmac_mdio_bus_data {
	int (*phy_reset)(void *priv);
	unsigned int phy_mask;
	int *irqs;
	int probed_phy_irq;
 };

Where:
 o phy_reset: hook to reset the phy device attached to the bus.
 o phy_mask: phy mask passed when register the MDIO bus within the driver.
 o irqs: list of IRQs, one per PHY.
 o probed_phy_irq: if irqs is NULL, use this for probed PHY.

For DMA engine we have the following internal fields that should be
tuned according to the HW capabilities.

struct stmmac_dma_cfg {
	int pbl;
	int txpbl;
	int rxpbl;
	bool pblx8;
	int fixed_burst;
	int mixed_burst;
	bool aal;
};

Where:
 o pbl: Programmable Burst Length (tx and rx)
 o txpbl: Transmit Programmable Burst Length. Only for GMAC and newer.
	 If set, DMA tx will use this value rather than pbl.
 o rxpbl: Receive Programmable Burst Length. Only for GMAC and newer.
	 If set, DMA rx will use this value rather than pbl.
 o pblx8: Enable 8xPBL (4xPBL for core rev < 3.50). Enabled by default.
 o fixed_burst: program the DMA to use the fixed burst mode
 o mixed_burst: program the DMA to use the mixed burst mode
 o aal: Address-Aligned Beats

---

Below an example how the structures above are using on ST platforms.

 static struct plat_stmmacenet_data stxYYY_ethernet_platform_data = {
	.has_gmac = 0,
	.enh_desc = 0,
	.fix_mac_speed = stxYYY_ethernet_fix_mac_speed,
				|
				|-> to write an internal syscfg
				|   on this platform when the
				|   link speed changes from 10 to
				|   100 and viceversa
	.init = &stmmac_claim_resource,
				|
				|-> On ST SoC this calls own "PAD"
				|   manager framework to claim
				|   all the resources necessary
				|   (GPIO ...). The .custom_cfg field
				|   is used to pass a custom config.
};

Below the usage of the stmmac_mdio_bus_data: on this SoC, in fact,
there are two MAC cores: one MAC is for MDIO Bus/PHY emulation
with fixed_link support.

static struct stmmac_mdio_bus_data stmmac1_mdio_bus = {
	.phy_reset = phy_reset;
		|
		|-> function to provide the phy_reset on this board
	.phy_mask = 0,
};

static struct fixed_phy_status stmmac0_fixed_phy_status = {
	.link = 1,
	.speed = 100,
	.duplex = 1,
};

During the board's device_init we can configure the first
MAC for fixed_link by calling:
  fixed_phy_add(PHY_POLL, 1, &stmmac0_fixed_phy_status, -1);
and the second one, with a real PHY device attached to the bus,
by using the stmmac_mdio_bus_data structure (to provide the id, the
reset procedure etc).

Note that, starting from new chips, where it is available the HW capability
register, many configurations are discovered at run-time for example to
understand if EEE, HW csum, PTP, enhanced descriptor etc are actually
available. As strategy adopted in this driver, the information from the HW
capability register can replace what has been passed from the platform.

4.10) Device-tree support.

Please see the following document:
	Documentation/devicetree/bindings/net/stmmac.txt

4.11) This is a summary of the content of some relevant files:
 o stmmac_main.c: implements the main network device driver;
 o stmmac_mdio.c: provides MDIO functions;
 o stmmac_pci: this is the PCI driver;
 o stmmac_platform.c: this the platform driver (OF supported);
 o stmmac_ethtool.c: implements the ethtool support;
 o stmmac.h: private driver structure;
 o common.h: common definitions and VFTs;
 o mmc_core.c/mmc.h: Management MAC Counters;
 o stmmac_hwtstamp.c: HW timestamp support for PTP;
 o stmmac_ptp.c: PTP 1588 clock;
 o stmmac_pcs.h: Physical Coding Sublayer common implementation;
 o dwmac-<XXX>.c: these are for the platform glue-logic file; e.g. dwmac-sti.c
   for STMicroelectronics SoCs.

- GMAC 3.x
 o descs.h: descriptor structure definitions;
 o dwmac1000_core.c: dwmac GiGa core functions;
 o dwmac1000_dma.c: dma functions for the GMAC chip;
 o dwmac1000.h: specific header file for the dwmac GiGa;
 o dwmac100_core: dwmac 100 core code;
 o dwmac100_dma.c: dma functions for the dwmac 100 chip;
 o dwmac1000.h: specific header file for the MAC;
 o dwmac_lib.c: generic DMA functions;
 o enh_desc.c: functions for handling enhanced descriptors;
 o norm_desc.c: functions for handling normal descriptors;
 o chain_mode.c/ring_mode.c:: functions to manage RING/CHAINED modes;

- GMAC4.x generation
 o dwmac4_core.c: dwmac GMAC4.x core functions;
 o dwmac4_desc.c: functions for handling GMAC4.x descriptors;
 o dwmac4_descs.h: descriptor definitions;
 o dwmac4_dma.c: dma functions for the GMAC4.x chip;
 o dwmac4_dma.h: dma definitions for the GMAC4.x chip;
 o dwmac4.h: core definitions for the GMAC4.x chip;
 o dwmac4_lib.c: generic GMAC4.x functions;

4.12) TSO support (GMAC4.x)

TSO (Tcp Segmentation Offload) feature is supported by GMAC 4.x chip family.
When a packet is sent through TCP protocol, the TCP stack ensures that
the SKB provided to the low level driver (stmmac in our case) matches with
the maximum frame len (IP header + TCP header + payload <= 1500 bytes (for
MTU set to 1500)). It means that if an application using TCP want to send a
packet which will have a length (after adding headers) > 1514 the packet
will be split in several TCP packets: The data payload is split and headers
(TCP/IP ..) are added. It is done by software.

When TSO is enabled, the TCP stack doesn't care about the maximum frame
length and provide SKB packet to stmmac as it is. The GMAC IP will have to
perform the segmentation by it self to match with maximum frame length.

This feature can be enabled in device tree through "snps,tso" entry.

5) Debug Information

The driver exports many information i.e. internal statistics,
debug information, MAC and DMA registers etc.

These can be read in several ways depending on the
type of the information actually needed.

For example a user can be use the ethtool support
to get statistics: e.g. using: ethtool -S ethX
(that shows the Management counters (MMC) if supported)
or sees the MAC/DMA registers: e.g. using: ethtool -d ethX

Compiling the Kernel with CONFIG_DEBUG_FS the driver will export the following
debugfs entries:

/sys/kernel/debug/stmmaceth/descriptors_status
  To show the DMA TX/RX descriptor rings

Developer can also use the "debug" module parameter to get further debug
information (please see: NETIF Msg Level).

6) Energy Efficient Ethernet

Energy Efficient Ethernet(EEE) enables IEEE 802.3 MAC sublayer along
with a family of Physical layer to operate in the Low power Idle(LPI)
mode. The EEE mode supports the IEEE 802.3 MAC operation at 100Mbps,
1000Mbps & 10Gbps.

The LPI mode allows power saving by switching off parts of the
communication device functionality when there is no data to be
transmitted & received. The system on both the side of the link can
disable some functionalities & save power during the period of low-link
utilization. The MAC controls whether the system should enter or exit
the LPI mode & communicate this to PHY.

As soon as the interface is opened, the driver verifies if the EEE can
be supported. This is done by looking at both the DMA HW capability
register and the PHY devices MCD registers.
To enter in Tx LPI mode the driver needs to have a software timer
that enable and disable the LPI mode when there is nothing to be
transmitted.

7) Precision Time Protocol (PTP)
The driver supports the IEEE 1588-2002, Precision Time Protocol (PTP),
which enables precise synchronization of clocks in measurement and
control systems implemented with technologies such as network
communication.

In addition to the basic timestamp features mentioned in IEEE 1588-2002
Timestamps, new GMAC cores support the advanced timestamp features.
IEEE 1588-2008 that can be enabled when configure the Kernel.

8) SGMII/RGMII support
New GMAC devices provide own way to manage RGMII/SGMII.
This information is available at run-time by looking at the
HW capability register. This means that the stmmac can manage
auto-negotiation and link status w/o using the PHYLIB stuff.
In fact, the HW provides a subset of extended registers to
restart the ANE, verify Full/Half duplex mode and Speed.
Thanks to these registers, it is possible to look at the
Auto-negotiated Link Parter Ability.