Commit 2f07c05f authored by Luca Ceresoli's avatar Luca Ceresoli Committed by Wolfram Sang
Browse files

docs: i2c: call it "I2C" consistently 



Uppercase "I2C" is used almost everywhere in the docs, but the lowercase
version "i2c" is used somewhere. Use the uppercase form consistently.

Signed-off-by: default avatarLuca Ceresoli <luca@lucaceresoli.net>
Acked-by: default avatarPeter Rosin <peda@axentia.se>
Reviewed-by: default avatarJean Delvare <jdelvare@suse.de>
Signed-off-by: default avatarWolfram Sang <wsa@the-dreams.de>
parent 020bc5b9

Documentation/i2c/dev-interface.rst

+9 −9
Original line number Diff line number Diff line
@@ -2,26 +2,26 @@ 
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>
@@ -66,7 +66,7 @@ the device supports them. Both are illustrated below:: 
  /* 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 */

  }
@@ -79,12 +79,12 @@ the device supports them. Both are illustrated below:: 
  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 */

  }
@@ -144,7 +144,7 @@ The following IOCTLs are defined: 
  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.

Documentation/i2c/dma-considerations.rst

+1 −1
Original line number Diff line number Diff line
@@ -2,7 +2,7 @@ 
Linux I2C and DMA

=================



Given that i2c is a low-speed bus, over which the majority of messages

Given that I2C is a low-speed bus, over which the majority of messages

transferred are small, it is not considered a prime user of DMA access. At this

time of writing, only 10% of I2C bus master drivers have DMA support

implemented. And the vast majority of transactions are so small that setting up

Documentation/i2c/i2c-protocol.rst

+2 −2
Original line number Diff line number Diff line
@@ -2,7 +2,7 @@ 
I2C Protocol

============



This document describes the i2c protocol. Or will, when it is finished :-)

This document describes the I2C protocol. Or will, when it is finished :-)



Key to symbols

==============
@@ -57,7 +57,7 @@ Modified transactions 
=====================



The following modifications to the I2C protocol can also be generated by

setting these flags for i2c messages. With the exception of I2C_M_NOSTART, they

setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they

are usually only needed to work around device issues:



I2C_M_IGNORE_NAK:

Documentation/i2c/i2c-topology.rst

+33 −33
Original line number Diff line number Diff line
@@ -2,8 +2,8 @@ 
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.
@@ -11,20 +11,20 @@ than a straight-forward i2c bus with one adapter and one or more devices. 
   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
@@ -34,7 +34,7 @@ optional) deselect is called after the transfer. 
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:
@@ -45,7 +45,7 @@ In drivers/i2c/muxes/: 
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
@@ -54,7 +54,7 @@ i2c-mux-pca9541 Parent-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

======================    =============================================
@@ -83,9 +83,9 @@ Mux-locked muxes 
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.
@@ -109,14 +109,14 @@ ML2. It is not safe to build arbitrary topologies with two (or more) 


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.

==== =====================================================================
@@ -137,14 +137,14 @@ Mux-locked Example 


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.
@@ -159,8 +159,8 @@ Parent-locked muxes 


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.
@@ -169,12 +169,12 @@ PL1. If you build a topology with a parent-locked mux being the child 
     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.
@@ -197,15 +197,15 @@ Parent-locked Example 


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.
@@ -240,7 +240,7 @@ and specifically when M2 requests its parent to lock, M1 passes 
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.
@@ -286,14 +286,14 @@ point. 


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.

Documentation/i2c/instantiating-devices.rst

+1 −1
Original line number Diff line number Diff line
@@ -188,7 +188,7 @@ destroyed automatically when the driver which detected them is removed, 
or when the underlying I2C bus is itself destroyed, whichever happens

first.



Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6

Those of you familiar with the I2C subsystem of 2.4 kernels and early 2.6

kernels will find out that this method 3 is essentially similar to what

was done there. Two significant differences are:

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