I2C Device Interface
====================
-Usually, i2c devices are controlled by a kernel driver. But it is also
+Usually, I2C devices are controlled by a kernel driver. But it is also
possible to access all devices on an adapter from userspace, through
the /dev interface. You need to load module i2c-dev for this.
-Each registered i2c adapter gets a number, counting from 0. You can
+Each registered I2C adapter gets a number, counting from 0. You can
examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all
-i2c adapters present on your system at a given time. i2cdetect is part of
+I2C adapters present on your system at a given time. i2cdetect is part of
the i2c-tools package.
I2C device files are character device files with major device number 89
and a minor device number corresponding to the number assigned as
explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
-i2c-10, ...). All 256 minor device numbers are reserved for i2c.
+i2c-10, ...). All 256 minor device numbers are reserved for I2C.
C example
=========
-So let's say you want to access an i2c adapter from a C program.
+So let's say you want to access an I2C adapter from a C program.
First, you need to include these two headers::
#include <linux/i2c-dev.h>
/* Using SMBus commands */
res = i2c_smbus_read_word_data(file, reg);
if (res < 0) {
- /* ERROR HANDLING: i2c transaction failed */
+ /* ERROR HANDLING: I2C transaction failed */
} else {
/* res contains the read word */
}
buf[1] = 0x43;
buf[2] = 0x65;
if (write(file, buf, 3) != 3) {
- /* ERROR HANDLING: i2c transaction failed */
+ /* ERROR HANDLING: I2C transaction failed */
}
/* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
if (read(file, buf, 1) != 1) {
- /* ERROR HANDLING: i2c transaction failed */
+ /* ERROR HANDLING: I2C transaction failed */
} else {
/* buf[0] contains the read byte */
}
If possible, use the provided ``i2c_smbus_*`` methods described below instead
of issuing direct ioctls.
-You can do plain i2c transactions by using read(2) and write(2) calls.
+You can do plain I2C transactions by using read(2) and write(2) calls.
You do not need to pass the address byte; instead, set it through
ioctl I2C_SLAVE before you try to access the device.
I2C topology
============
-There are a couple of reasons for building more complex i2c topologies
-than a straight-forward i2c bus with one adapter and one or more devices.
+There are a couple of reasons for building more complex I2C topologies
+than a straight-forward I2C bus with one adapter and one or more devices.
1. A mux may be needed on the bus to prevent address collisions.
may be needed to determine if it is ok to access the bus.
3. A device (particularly RF tuners) may want to avoid the digital noise
- from the i2c bus, at least most of the time, and sits behind a gate
+ from the I2C bus, at least most of the time, and sits behind a gate
that has to be operated before the device can be accessed.
Etc
===
-These constructs are represented as i2c adapter trees by Linux, where
+These constructs are represented as I2C adapter trees by Linux, where
each adapter has a parent adapter (except the root adapter) and zero or
more child adapters. The root adapter is the actual adapter that issues
-i2c transfers, and all adapters with a parent are part of an "i2c-mux"
+I2C transfers, and all adapters with a parent are part of an "i2c-mux"
object (quoted, since it can also be an arbitrator or a gate).
Depending of the particular mux driver, something happens when there is
-an i2c transfer on one of its child adapters. The mux driver can
+an I2C transfer on one of its child adapters. The mux driver can
obviously operate a mux, but it can also do arbitration with an external
bus master or open a gate. The mux driver has two operations for this,
select and deselect. select is called before the transfer and (the
Locking
=======
-There are two variants of locking available to i2c muxes, they can be
+There are two variants of locking available to I2C muxes, they can be
mux-locked or parent-locked muxes. As is evident from below, it can be
useful to know if a mux is mux-locked or if it is parent-locked. The
following list was correct at the time of writing:
i2c-arb-gpio-challenge Parent-locked
i2c-mux-gpio Normally parent-locked, mux-locked iff
all involved gpio pins are controlled by the
- same i2c root adapter that they mux.
+ same I2C root adapter that they mux.
i2c-mux-gpmux Normally parent-locked, mux-locked iff
specified in device-tree.
i2c-mux-ltc4306 Mux-locked
i2c-mux-pca954x Parent-locked
i2c-mux-pinctrl Normally parent-locked, mux-locked iff
all involved pinctrl devices are controlled
- by the same i2c root adapter that they mux.
+ by the same I2C root adapter that they mux.
i2c-mux-reg Parent-locked
====================== =============================================
Mux-locked muxes does not lock the entire parent adapter during the
full select-transfer-deselect transaction, only the muxes on the parent
adapter are locked. Mux-locked muxes are mostly interesting if the
-select and/or deselect operations must use i2c transfers to complete
+select and/or deselect operations must use I2C transfers to complete
their tasks. Since the parent adapter is not fully locked during the
-full transaction, unrelated i2c transfers may interleave the different
+full transaction, unrelated I2C transfers may interleave the different
stages of the transaction. This has the benefit that the mux driver
may be easier and cleaner to implement, but it has some caveats.
ML3. A mux-locked mux cannot be used by a driver for auto-closing
gates/muxes, i.e. something that closes automatically after a given
- number (one, in most cases) of i2c transfers. Unrelated i2c transfers
+ number (one, in most cases) of I2C transfers. Unrelated I2C transfers
may creep in and close prematurely.
-ML4. If any non-i2c operation in the mux driver changes the i2c mux state,
+ML4. If any non-I2C operation in the mux driver changes the I2C mux state,
the driver has to lock the root adapter during that operation.
Otherwise garbage may appear on the bus as seen from devices
- behind the mux, when an unrelated i2c transfer is in flight during
- the non-i2c mux-changing operation.
+ behind the mux, when an unrelated I2C transfer is in flight during
+ the non-I2C mux-changing operation.
==== =====================================================================
When there is an access to D1, this happens:
- 1. Someone issues an i2c-transfer to D1.
+ 1. Someone issues an I2C-transfer to D1.
2. M1 locks muxes on its parent (the root adapter in this case).
3. M1 calls ->select to ready the mux.
- 4. M1 (presumably) does some i2c-transfers as part of its select.
- These transfers are normal i2c-transfers that locks the parent
+ 4. M1 (presumably) does some I2C-transfers as part of its select.
+ These transfers are normal I2C-transfers that locks the parent
adapter.
- 5. M1 feeds the i2c-transfer from step 1 to its parent adapter as a
- normal i2c-transfer that locks the parent adapter.
+ 5. M1 feeds the I2C-transfer from step 1 to its parent adapter as a
+ normal I2C-transfer that locks the parent adapter.
6. M1 calls ->deselect, if it has one.
7. Same rules as in step 4, but for ->deselect.
8. M1 unlocks muxes on its parent.
Parent-locked muxes lock the parent adapter during the full select-
transfer-deselect transaction. The implication is that the mux driver
-has to ensure that any and all i2c transfers through that parent
-adapter during the transaction are unlocked i2c transfers (using e.g.
+has to ensure that any and all I2C transfers through that parent
+adapter during the transaction are unlocked I2C transfers (using e.g.
__i2c_transfer), or a deadlock will follow. There are a couple of
caveats.
of another mux, this might break a possible assumption from the
child mux that the root adapter is unused between its select op
and the actual transfer (e.g. if the child mux is auto-closing
- and the parent mux issus i2c-transfers as part of its select).
+ and the parent mux issus I2C-transfers as part of its select).
This is especially the case if the parent mux is mux-locked, but
it may also happen if the parent mux is parent-locked.
PL2. If select/deselect calls out to other subsystems such as gpio,
- pinctrl, regmap or iio, it is essential that any i2c transfers
+ pinctrl, regmap or iio, it is essential that any I2C transfers
caused by these subsystems are unlocked. This can be convoluted to
accomplish, maybe even impossible if an acceptably clean solution
is sought.
When there is an access to D1, this happens:
- 1. Someone issues an i2c-transfer to D1.
+ 1. Someone issues an I2C-transfer to D1.
2. M1 locks muxes on its parent (the root adapter in this case).
3. M1 locks its parent adapter.
4. M1 calls ->select to ready the mux.
- 5. If M1 does any i2c-transfers (on this root adapter) as part of
- its select, those transfers must be unlocked i2c-transfers so
+ 5. If M1 does any I2C-transfers (on this root adapter) as part of
+ its select, those transfers must be unlocked I2C-transfers so
that they do not deadlock the root adapter.
- 6. M1 feeds the i2c-transfer from step 1 to the root adapter as an
- unlocked i2c-transfer, so that it does not deadlock the parent
+ 6. M1 feeds the I2C-transfer from step 1 to the root adapter as an
+ unlocked I2C-transfer, so that it does not deadlock the parent
adapter.
7. M1 calls ->deselect, if it has one.
8. Same rules as in step 5, but for ->deselect.
the buck to the root adapter).
This topology is bad if M2 is an auto-closing mux and M1->select
-issues any unlocked i2c transfers on the root adapter that may leak
+issues any unlocked I2C transfers on the root adapter that may leak
through and be seen by the M2 adapter, thus closing M2 prematurely.
This kind of topology is generally not suitable and should probably
be avoided. The reason is that M2 probably assumes that there will
-be no i2c transfers during its calls to ->select and ->deselect, and
+be no I2C transfers during its calls to ->select and ->deselect, and
if there are, any such transfers might appear on the slave side of M2
-as partial i2c transfers, i.e. garbage or worse. This might cause
+as partial I2C transfers, i.e. garbage or worse. This might cause
device lockups and/or other problems.
The topology is especially troublesome if M2 is an auto-closing
mux. In that case, any interleaved accesses to D4 might close M2
-prematurely, as might any i2c-transfers part of M1->select.
+prematurely, as might any I2C-transfers part of M1->select.
But if M2 is not making the above stated assumption, and if M2 is not
auto-closing, the topology is fine.
projects / platform / kernel / linux-starfive.git / commitdiff