stmmac: add documentation for the driver.

Add Documentation/networking/stmmac.txt for the
stmmac network driver.
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/stmmac.txt

+       STMicroelectronics 10/100/1000 Synopsys Ethernet driver
+
+Copyright (C) 2007-2010  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); it has been fully tested on STLinux platforms.
+
+Currently this network device driver is for all STM embedded MAC/GMAC
+(7xxx SoCs).
+
+DWC Ether MAC 10/100/1000 Universal version 3.41a and DWC Ether MAC 10/100
+Universal version 4.0 have been used for developing the first code
+implementation.
+
+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)
+
+2) Driver parameters list:
+	debug: message level (0: no output, 16: all);
+	phyaddr: to manually provide the physical address to the PHY device;
+	dma_rxsize: DMA rx ring size;
+	dma_txsize: DMA tx ring size;
+	buf_sz: DMA buffer size;
+	tc: control the HW FIFO threshold;
+	tx_coe: Enable/Disable Tx Checksum Offload engine;
+	watchdog: transmit timeout (in milliseconds);
+	flow_ctrl: Flow control ability [on/off];
+	pause: Flow Control Pause Time;
+	tmrate: timer period (only if timer optimisation is configured).
+
+3) Command line options
+Driver parameters can be also passed in command line by using:
+	stmmaceth=dma_rxsize:128,dma_txsize:512
+
+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.
+Once the controller has finished transmitting the packet, an interrupt is
+triggered; So the driver will be able to release the socket buffers.
+By default, the driver sets the NETIF_F_SG bit in the features field of the
+net_device structure enabling the scatter/gather feature.
+
+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) Timer-Driver Interrupt
+Instead of having the device that asynchronously notifies the frame receptions, the
+driver configures a timer to generate an interrupt at regular intervals.
+Based on the granularity of the timer, the frames that are received by the device
+will experience different levels of latency. Some NICs have dedicated timer
+device to perform this task. STMMAC can use either the RTC device or the TMU
+channel 2  on STLinux platforms.
+The timers frequency can be passed to the driver as parameter; when change it,
+take care of both hardware capability and network stability/performance impact.
+Several performance tests on STM platforms showed this optimisation allows to spare
+the CPU while having the maximum throughput.
+
+4.4) WOL
+Wake up on Lan feature through Magic Frame is only supported for the GMAC
+core.
+
+4.5) DMA descriptors
+Driver handles both normal and enhanced descriptors. The latter has been only
+tested on DWC Ether MAC 10/100/1000 Universal version 3.41a.
+
+4.6) Ethtool support
+Ethtool is supported. Driver statistics and internal errors can be taken using:
+ethtool -S ethX command. It is possible to dump registers etc.
+
+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 PAL to work with PHY and GPHY devices.
+
+4.9) Platform information
+Several information came from the platform; please refer to the
+driver's Header file in include/linux directory.
+
+struct plat_stmmacenet_data {
+        int bus_id;
+        int pbl;
+        int has_gmac;
+        void (*fix_mac_speed)(void *priv, unsigned int speed);
+        void (*bus_setup)(unsigned long ioaddr);
+#ifdef CONFIG_STM_DRIVERS
+        struct stm_pad_config *pad_config;
+#endif
+        void *bsp_priv;
+};
+
+Where:
+- pbl (Programmable Burst Length) is maximum number of
+  beats to be transferred in one DMA transaction.
+  GMAC also enables the 4xPBL by default.
+- fix_mac_speed and bus_setup are used to configure internal target
+  registers (on STM platforms);
+- has_gmac: GMAC core is on board (get it at run-time in the next step);
+- bus_id: bus identifier.
+
+struct plat_stmmacphy_data {
+        int bus_id;
+        int phy_addr;
+        unsigned int phy_mask;
+        int interface;
+        int (*phy_reset)(void *priv);
+        void *priv;
+};
+
+Where:
+- bus_id: bus identifier;
+- phy_addr: physical address used for the attached phy device;
+            set it to -1 to get it at run-time;
+- interface: physical MII interface mode;
+- phy_reset: hook to reset HW function.
+
+TODO:
+- Continue to make the driver more generic and suitable for other Synopsys
+  Ethernet controllers used on other architectures (i.e. ARM).
+- 10G controllers are not supported.
+- MAC uses Normal descriptors and GMAC uses enhanced ones.
+  This is a limit that should be reviewed. MAC could want to
+  use the enhanced structure.
+- Checksumming: Rx/Tx csum is done in HW in case of GMAC only.
+- Review the timer optimisation code to use an embedded device that seems to be
+  available in new chip generations.