Commit 92d5d643 authored by Luca Ceresoli's avatar Luca Ceresoli Committed by Wolfram Sang
Browse files

docs: i2c: i2c-topology: reorder sections more logically 



The sequence of sections is a bit confusing here:

 * we list the mux locking scheme for existing drivers before introducing
   what mux locking schemes are
 * we list the caveats for each locking scheme (which are tricky) before
   the example of the simple use case

Restructure it entirely with the following logic:

 * Intro ("I2C muxes and complex topologies")
 * Locking
   - mux-locked
     - example
     - caveats
   - parent-locked
     - example
     - caveats
 * Complex examples
 * Mux type of existing device drivers

While there, also apply some other improvements:

 * convert the caveat list from a table (with only one column carrying
   content) to a bullet list.
 * add a small introductory text to bridge the gap from listing the use
   cases to telling about the hardware components to handle them and then
   the device drivers that implement those.
 * make empty lines usage more uniform

Signed-off-by: default avatarLuca Ceresoli <luca.ceresoli@bootlin.com>
Acked-by: default avatarPeter Rosin <peda@axentia.se>
Signed-off-by: default avatarWolfram Sang <wsa@kernel.org>
parent 12c035a1

Documentation/i2c/i2c-topology.rst

+114 −97
Original line number Diff line number Diff line
@@ -16,7 +16,10 @@ Some example use cases are: 
   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.



These constructs are represented as I2C adapter trees by Linux, where

Several types of hardware components such as I2C muxes, I2C gates and I2C

arbitrators allow to handle such needs.



These components 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"
@@ -34,46 +37,7 @@ Locking 
=======



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:



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.

i2c-mux-gpmux             Normally parent-locked, mux-locked iff

                          specified in device-tree.

i2c-mux-ltc4306           Mux-locked

i2c-mux-mlxcpld           Parent-locked

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.

i2c-mux-reg               Parent-locked

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



In drivers/iio/:



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

gyro/mpu3050              Mux-locked

imu/inv_mpu6050/          Mux-locked

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



In drivers/media/:



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

dvb-frontends/lgdt3306a   Mux-locked

dvb-frontends/m88ds3103   Parent-locked

dvb-frontends/rtl2830     Parent-locked

dvb-frontends/rtl2832     Mux-locked

dvb-frontends/si2168      Mux-locked

usb/cx231xx/              Parent-locked

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

mux-locked or parent-locked muxes.





Mux-locked muxes
@@ -88,40 +52,8 @@ 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.



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

ML1. If you build a topology with a mux-locked mux being the parent

     of a parent-locked mux, this might break the expectation from the

     parent-locked mux that the root adapter is locked during the

     transaction.



ML2. It is not safe to build arbitrary topologies with two (or more)

     mux-locked muxes that are not siblings, when there are address

     collisions between the devices on the child adapters of these

     non-sibling muxes.



     I.e. the select-transfer-deselect transaction targeting e.g. device

     address 0x42 behind mux-one may be interleaved with a similar

     operation targeting device address 0x42 behind mux-two. The

     intent with such a topology would in this hypothetical example

     be that mux-one and mux-two should not be selected simultaneously,

     but mux-locked muxes do not guarantee that in all topologies.



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

     may creep in and close prematurely.



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.

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





Mux-locked Example

------------------



~~~~~~~~~~~~~~~~~~



::
@@ -152,6 +84,43 @@ This means that accesses to D2 are lockout out for the full duration 
of the entire operation. But accesses to D3 are possibly interleaved

at any point.



Mux-locked caveats

~~~~~~~~~~~~~~~~~~



When using a mux-locked mux, be aware of the following restrictions:



[ML1]

  If you build a topology with a mux-locked mux being the parent

  of a parent-locked mux, this might break the expectation from the

  parent-locked mux that the root adapter is locked during the

  transaction.



[ML2]

  It is not safe to build arbitrary topologies with two (or more)

  mux-locked muxes that are not siblings, when there are address

  collisions between the devices on the child adapters of these

  non-sibling muxes.



  I.e. the select-transfer-deselect transaction targeting e.g. device

  address 0x42 behind mux-one may be interleaved with a similar

  operation targeting device address 0x42 behind mux-two. The

  intent with such a topology would in this hypothetical example

  be that mux-one and mux-two should not be selected simultaneously,

  but mux-locked muxes do not guarantee that in all topologies.



[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

  may creep in and close prematurely.



[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.





Parent-locked muxes

-------------------
@@ -160,28 +129,10 @@ 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.

__i2c_transfer), or a deadlock will follow. There are a couple of

caveats.



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

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 issues 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

     caused by these subsystems are unlocked. This can be convoluted to

     accomplish, maybe even impossible if an acceptably clean solution

     is sought.

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



__i2c_transfer), or a deadlock will follow.



Parent-locked Example

---------------------

~~~~~~~~~~~~~~~~~~~~~



::
@@ -211,10 +162,30 @@ When there is an access to D1, this happens: 
 9.  M1 unlocks its parent adapter.

 10. M1 unlocks muxes on its parent.





This means that accesses to both D2 and D3 are locked out for the full

duration of the entire operation.



Parent-locked Caveats

~~~~~~~~~~~~~~~~~~~~~



When using a parent-locked mux, be aware of the following restrictions:



[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 issues 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

  caused by these subsystems are unlocked. This can be convoluted to

  accomplish, maybe even impossible if an acceptably clean solution

  is sought.





Complex Examples

================
@@ -260,8 +231,10 @@ This is a good topology:: 
When device D1 is accessed, accesses to D2 are locked out for the

full duration of the operation (muxes on the top child adapter of M1

are locked). But accesses to D3 and D4 are possibly interleaved at

any point. Accesses to D3 locks out D1 and D2, but accesses to D4

are still possibly interleaved.

any point.



Accesses to D3 locks out D1 and D2, but accesses to D4 are still possibly

interleaved.





Mux-locked mux as parent of parent-locked mux
@@ -393,3 +366,47 @@ This is a good topology:: 
When D1 or D2 are accessed, accesses to D3 and D4 are locked out while

accesses to D5 may interleave. When D3 or D4 are accessed, accesses to

all other devices are locked out.





Mux type of existing device drivers

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



Whether a device is mux-locked or parent-locked depends on its

implementation. The following list was correct at the time of writing:



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.

i2c-mux-gpmux             Normally parent-locked, mux-locked iff

                          specified in device-tree.

i2c-mux-ltc4306           Mux-locked

i2c-mux-mlxcpld           Parent-locked

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.

i2c-mux-reg               Parent-locked

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



In drivers/iio/:



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

gyro/mpu3050              Mux-locked

imu/inv_mpu6050/          Mux-locked

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



In drivers/media/:



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

dvb-frontends/lgdt3306a   Mux-locked

dvb-frontends/m88ds3103   Parent-locked

dvb-frontends/rtl2830     Parent-locked

dvb-frontends/rtl2832     Mux-locked

dvb-frontends/si2168      Mux-locked

usb/cx231xx/              Parent-locked

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