tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1.. SPDX-License-Identifier: GPL-2.0
2
3================
4CAIF SPI porting
5================
6
7CAIF SPI basics
8===============
9
10Running CAIF over SPI needs some extra setup, owing to the nature of SPI.
11Two extra GPIOs have been added in order to negotiate the transfers
12between the master and the slave. The minimum requirement for running
13CAIF over SPI is a SPI slave chip and two GPIOs (more details below).
14Please note that running as a slave implies that you need to keep up
15with the master clock. An overrun or underrun event is fatal.
16
17CAIF SPI framework
18==================
19
20To make porting as easy as possible, the CAIF SPI has been divided in
21two parts. The first part (called the interface part) deals with all
22generic functionality such as length framing, SPI frame negotiation
23and SPI frame delivery and transmission. The other part is the CAIF
24SPI slave device part, which is the module that you have to write if
25you want to run SPI CAIF on a new hardware. This part takes care of
26the physical hardware, both with regard to SPI and to GPIOs.
27
28- Implementing a CAIF SPI device:
29
30 - Functionality provided by the CAIF SPI slave device:
31
32 In order to implement a SPI device you will, as a minimum,
33 need to implement the following
34 functions:
35
36 ::
37
38 int (*init_xfer) (struct cfspi_xfer * xfer, struct cfspi_dev *dev):
39
40 This function is called by the CAIF SPI interface to give
41 you a chance to set up your hardware to be ready to receive
42 a stream of data from the master. The xfer structure contains
43 both physical and logical addresses, as well as the total length
44 of the transfer in both directions.The dev parameter can be used
45 to map to different CAIF SPI slave devices.
46
47 ::
48
49 void (*sig_xfer) (bool xfer, struct cfspi_dev *dev):
50
51 This function is called by the CAIF SPI interface when the output
52 (SPI_INT) GPIO needs to change state. The boolean value of the xfer
53 variable indicates whether the GPIO should be asserted (HIGH) or
54 deasserted (LOW). The dev parameter can be used to map to different CAIF
55 SPI slave devices.
56
57 - Functionality provided by the CAIF SPI interface:
58
59 ::
60
61 void (*ss_cb) (bool assert, struct cfspi_ifc *ifc);
62
63 This function is called by the CAIF SPI slave device in order to
64 signal a change of state of the input GPIO (SS) to the interface.
65 Only active edges are mandatory to be reported.
66 This function can be called from IRQ context (recommended in order
67 not to introduce latency). The ifc parameter should be the pointer
68 returned from the platform probe function in the SPI device structure.
69
70 ::
71
72 void (*xfer_done_cb) (struct cfspi_ifc *ifc);
73
74 This function is called by the CAIF SPI slave device in order to
75 report that a transfer is completed. This function should only be
76 called once both the transmission and the reception are completed.
77 This function can be called from IRQ context (recommended in order
78 not to introduce latency). The ifc parameter should be the pointer
79 returned from the platform probe function in the SPI device structure.
80
81 - Connecting the bits and pieces:
82
83 - Filling in the SPI slave device structure:
84
85 Connect the necessary callback functions.
86
87 Indicate clock speed (used to calculate toggle delays).
88
89 Chose a suitable name (helps debugging if you use several CAIF
90 SPI slave devices).
91
92 Assign your private data (can be used to map to your
93 structure).
94
95 - Filling in the SPI slave platform device structure:
96
97 Add name of driver to connect to ("cfspi_sspi").
98
99 Assign the SPI slave device structure as platform data.
100
101Padding
102=======
103
104In order to optimize throughput, a number of SPI padding options are provided.
105Padding can be enabled independently for uplink and downlink transfers.
106Padding can be enabled for the head, the tail and for the total frame size.
107The padding needs to be correctly configured on both sides of the link.
108The padding can be changed via module parameters in cfspi_sspi.c or via
109the sysfs directory of the cfspi_sspi driver (before device registration).
110
111- CAIF SPI device template::
112
113 /*
114 * Copyright (C) ST-Ericsson AB 2010
115 * Author: Daniel Martensson / Daniel.Martensson@stericsson.com
116 * License terms: GNU General Public License (GPL), version 2.
117 *
118 */
119
120 #include <linux/init.h>
121 #include <linux/module.h>
122 #include <linux/device.h>
123 #include <linux/wait.h>
124 #include <linux/interrupt.h>
125 #include <linux/dma-mapping.h>
126 #include <net/caif/caif_spi.h>
127
128 MODULE_LICENSE("GPL");
129
130 struct sspi_struct {
131 struct cfspi_dev sdev;
132 struct cfspi_xfer *xfer;
133 };
134
135 static struct sspi_struct slave;
136 static struct platform_device slave_device;
137
138 static irqreturn_t sspi_irq(int irq, void *arg)
139 {
140 /* You only need to trigger on an edge to the active state of the
141 * SS signal. Once a edge is detected, the ss_cb() function should be
142 * called with the parameter assert set to true. It is OK
143 * (and even advised) to call the ss_cb() function in IRQ context in
144 * order not to add any delay. */
145
146 return IRQ_HANDLED;
147 }
148
149 static void sspi_complete(void *context)
150 {
151 /* Normally the DMA or the SPI framework will call you back
152 * in something similar to this. The only thing you need to
153 * do is to call the xfer_done_cb() function, providing the pointer
154 * to the CAIF SPI interface. It is OK to call this function
155 * from IRQ context. */
156 }
157
158 static int sspi_init_xfer(struct cfspi_xfer *xfer, struct cfspi_dev *dev)
159 {
160 /* Store transfer info. For a normal implementation you should
161 * set up your DMA here and make sure that you are ready to
162 * receive the data from the master SPI. */
163
164 struct sspi_struct *sspi = (struct sspi_struct *)dev->priv;
165
166 sspi->xfer = xfer;
167
168 return 0;
169 }
170
171 void sspi_sig_xfer(bool xfer, struct cfspi_dev *dev)
172 {
173 /* If xfer is true then you should assert the SPI_INT to indicate to
174 * the master that you are ready to receive the data from the master
175 * SPI. If xfer is false then you should de-assert SPI_INT to indicate
176 * that the transfer is done.
177 */
178
179 struct sspi_struct *sspi = (struct sspi_struct *)dev->priv;
180 }
181
182 static void sspi_release(struct device *dev)
183 {
184 /*
185 * Here you should release your SPI device resources.
186 */
187 }
188
189 static int __init sspi_init(void)
190 {
191 /* Here you should initialize your SPI device by providing the
192 * necessary functions, clock speed, name and private data. Once
193 * done, you can register your device with the
194 * platform_device_register() function. This function will return
195 * with the CAIF SPI interface initialized. This is probably also
196 * the place where you should set up your GPIOs, interrupts and SPI
197 * resources. */
198
199 int res = 0;
200
201 /* Initialize slave device. */
202 slave.sdev.init_xfer = sspi_init_xfer;
203 slave.sdev.sig_xfer = sspi_sig_xfer;
204 slave.sdev.clk_mhz = 13;
205 slave.sdev.priv = &slave;
206 slave.sdev.name = "spi_sspi";
207 slave_device.dev.release = sspi_release;
208
209 /* Initialize platform device. */
210 slave_device.name = "cfspi_sspi";
211 slave_device.dev.platform_data = &slave.sdev;
212
213 /* Register platform device. */
214 res = platform_device_register(&slave_device);
215 if (res) {
216 printk(KERN_WARNING "sspi_init: failed to register dev.\n");
217 return -ENODEV;
218 }
219
220 return res;
221 }
222
223 static void __exit sspi_exit(void)
224 {
225 platform_device_del(&slave_device);
226 }
227
228 module_init(sspi_init);
229 module_exit(sspi_exit);