1- CAIF SPI porting -
2
3- CAIF SPI basics:
4
5Running CAIF over SPI needs some extra setup, owing to the nature of SPI.
6Two extra GPIOs have been added in order to negotiate the transfers
7 between the master and the slave. The minimum requirement for running
8CAIF over SPI is a SPI slave chip and two GPIOs (more details below).
9Please note that running as a slave implies that you need to keep up
10with the master clock. An overrun or underrun event is fatal.
11
12- CAIF SPI framework:
13
14To make porting as easy as possible, the CAIF SPI has been divided in
15two parts. The first part (called the interface part) deals with all
16generic functionality such as length framing, SPI frame negotiation
17and SPI frame delivery and transmission. The other part is the CAIF
18SPI slave device part, which is the module that you have to write if
19you want to run SPI CAIF on a new hardware. This part takes care of
20the physical hardware, both with regard to SPI and to GPIOs.
21
22- Implementing a CAIF SPI device:
23
24	- Functionality provided by the CAIF SPI slave device:
25
26	In order to implement a SPI device you will, as a minimum,
27	need to implement the following
28	functions:
29
30	int (*init_xfer) (struct cfspi_xfer * xfer, struct cfspi_dev *dev):
31
32	This function is called by the CAIF SPI interface to give
33	you a chance to set up your hardware to be ready to receive
34	a stream of data from the master. The xfer structure contains
35	both physical and logical addresses, as well as the total length
36	of the transfer in both directions.The dev parameter can be used
37	to map to different CAIF SPI slave devices.
38
39	void (*sig_xfer) (bool xfer, struct cfspi_dev *dev):
40
41	This function is called by the CAIF SPI interface when the output
42	(SPI_INT) GPIO needs to change state. The boolean value of the xfer
43	variable indicates whether the GPIO should be asserted (HIGH) or
44	deasserted (LOW). The dev parameter can be used to map to different CAIF
45	SPI slave devices.
46
47	- Functionality provided by the CAIF SPI interface:
48
49	void (*ss_cb) (bool assert, struct cfspi_ifc *ifc);
50
51	This function is called by the CAIF SPI slave device in order to
52	signal a change of state of the input GPIO (SS) to the interface.
53	Only active edges are mandatory to be reported.
54	This function can be called from IRQ context (recommended in order
55	not to introduce latency). The ifc parameter should be the pointer
56	returned from the platform probe function in the SPI device structure.
57
58	void (*xfer_done_cb) (struct cfspi_ifc *ifc);
59
60	This function is called by the CAIF SPI slave device in order to
61	report that a transfer is completed. This function should only be
62	called once both the transmission and the reception are completed.
63	This function can be called from IRQ context (recommended in order
64	not to introduce latency). The ifc parameter should be the pointer
65	returned from the platform probe function in the SPI device structure.
66
67	- Connecting the bits and pieces:
68
69		- Filling in the SPI slave device structure:
70
71		Connect the necessary callback functions.
72		Indicate clock speed (used to calculate toggle delays).
73		Chose a suitable name (helps debugging if you use several CAIF
74		SPI slave devices).
75		Assign your private data (can be used to map to your structure).
76
77		- Filling in the SPI slave platform device structure:
78		Add name of driver to connect to ("cfspi_sspi").
79		Assign the SPI slave device structure as platform data.
80
81- Padding:
82
83In order to optimize throughput, a number of SPI padding options are provided.
84Padding can be enabled independently for uplink and downlink transfers.
85Padding can be enabled for the head, the tail and for the total frame size.
86The padding needs to be correctly configured on both sides of the link.
87The padding can be changed via module parameters in cfspi_sspi.c or via
88the sysfs directory of the cfspi_sspi driver (before device registration).
89
90- CAIF SPI device template:
91
92/*
93 *	Copyright (C) ST-Ericsson AB 2010
94 *	Author: Daniel Martensson / Daniel.Martensson@stericsson.com
95 *	License terms: GNU General Public License (GPL), version 2.
96 *
97 */
98
99#include <linux/init.h>
100#include <linux/module.h>
101#include <linux/device.h>
102#include <linux/wait.h>
103#include <linux/interrupt.h>
104#include <linux/dma-mapping.h>
105#include <net/caif/caif_spi.h>
106
107MODULE_LICENSE("GPL");
108
109struct sspi_struct {
110	struct cfspi_dev sdev;
111	struct cfspi_xfer *xfer;
112};
113
114static struct sspi_struct slave;
115static struct platform_device slave_device;
116
117static irqreturn_t sspi_irq(int irq, void *arg)
118{
119	/* You only need to trigger on an edge to the active state of the
120	 * SS signal. Once a edge is detected, the ss_cb() function should be
121	 * called with the parameter assert set to true. It is OK
122	 * (and even advised) to call the ss_cb() function in IRQ context in
123	 * order not to add any delay. */
124
125	return IRQ_HANDLED;
126}
127
128static void sspi_complete(void *context)
129{
130	/* Normally the DMA or the SPI framework will call you back
131	 * in something similar to this. The only thing you need to
132	 * do is to call the xfer_done_cb() function, providing the pointer
133	 * to the CAIF SPI interface. It is OK to call this function
134	 * from IRQ context. */
135}
136
137static int sspi_init_xfer(struct cfspi_xfer *xfer, struct cfspi_dev *dev)
138{
139	/* Store transfer info. For a normal implementation you should
140	 * set up your DMA here and make sure that you are ready to
141	 * receive the data from the master SPI. */
142
143	struct sspi_struct *sspi = (struct sspi_struct *)dev->priv;
144
145	sspi->xfer = xfer;
146
147	return 0;
148}
149
150void sspi_sig_xfer(bool xfer, struct cfspi_dev *dev)
151{
152	/* If xfer is true then you should assert the SPI_INT to indicate to
153	 * the master that you are ready to receive the data from the master
154	 * SPI. If xfer is false then you should de-assert SPI_INT to indicate
155	 * that the transfer is done.
156	 */
157
158	struct sspi_struct *sspi = (struct sspi_struct *)dev->priv;
159}
160
161static void sspi_release(struct device *dev)
162{
163	/*
164	 * Here you should release your SPI device resources.
165	 */
166}
167
168static int __init sspi_init(void)
169{
170	/* Here you should initialize your SPI device by providing the
171	 * necessary functions, clock speed, name and private data. Once
172	 * done, you can register your device with the
173	 * platform_device_register() function. This function will return
174	 * with the CAIF SPI interface initialized. This is probably also
175	 * the place where you should set up your GPIOs, interrupts and SPI
176	 * resources. */
177
178	int res = 0;
179
180	/* Initialize slave device. */
181	slave.sdev.init_xfer = sspi_init_xfer;
182	slave.sdev.sig_xfer = sspi_sig_xfer;
183	slave.sdev.clk_mhz = 13;
184	slave.sdev.priv = &slave;
185	slave.sdev.name = "spi_sspi";
186	slave_device.dev.release = sspi_release;
187
188	/* Initialize platform device. */
189	slave_device.name = "cfspi_sspi";
190	slave_device.dev.platform_data = &slave.sdev;
191
192	/* Register platform device. */
193	res = platform_device_register(&slave_device);
194	if (res) {
195		printk(KERN_WARNING "sspi_init: failed to register dev.\n");
196		return -ENODEV;
197	}
198
199	return res;
200}
201
202static void __exit sspi_exit(void)
203{
204	platform_device_del(&slave_device);
205}
206
207module_init(sspi_init);
208module_exit(sspi_exit);
209