================================
  I2C muxes and complex topologies
  ================================

  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.
  
  2. The bus may be accessible from some external bus master, and arbitration
     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

     that has to be operated before the device can be accessed.
  
  Etc

  ===

  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"

  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

  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
  optional) deselect is called after the transfer.
  
  
  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 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

  their tasks. Since the parent adapter is not fully locked during the

  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
       intension 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
  ------------------

  
  ::

                     .----------.     .--------.
      .--------.     |   mux-   |-----| dev D1 |
      |  root  |--+--|  locked  |     '--------'
      '--------'  |  |  mux M1  |--.  .--------.
                  |  '----------'  '--| dev D2 |
                  |  .--------.       '--------'
                  '--| dev D3 |
                     '--------'
  
  When there is an access to D1, this happens:

   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

      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.
  
  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.
  
  
  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.

  __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.

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

  
  
  Parent-locked Example
  ---------------------

  ::

                     .----------.     .--------.
      .--------.     |  parent- |-----| dev D1 |
      |  root  |--+--|  locked  |     '--------'
      '--------'  |  |  mux M1  |--.  .--------.
                  |  '----------'  '--| dev D2 |
                  |  .--------.       '--------'
                  '--| dev D3 |
                     '--------'
  
  When there is an access to D1, this happens:

   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

       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

       adapter.
   7.  M1 calls ->deselect, if it has one.
   8.  Same rules as in step 5, but for ->deselect.
   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.
  
  
  Complex Examples
  ================
  
  Parent-locked mux as parent of parent-locked mux
  ------------------------------------------------

  This is a useful topology, but it can be bad::

  
                     .----------.     .----------.     .--------.
      .--------.     |  parent- |-----|  parent- |-----| dev D1 |
      |  root  |--+--|  locked  |     |  locked  |     '--------'
      '--------'  |  |  mux M1  |--.  |  mux M2  |--.  .--------.
                  |  '----------'  |  '----------'  '--| dev D2 |
                  |  .--------.    |  .--------.       '--------'
                  '--| dev D4 |    '--| dev D3 |
                     '--------'       '--------'
  
  When any device is accessed, all other devices are locked out for
  the full duration of the operation (both muxes lock their parent,
  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

  through and be seen by the M2 adapter, thus closing M2 prematurely.
  
  
  Mux-locked mux as parent of mux-locked mux
  ------------------------------------------

  This is a good topology::

  
                     .----------.     .----------.     .--------.
      .--------.     |   mux-   |-----|   mux-   |-----| dev D1 |
      |  root  |--+--|  locked  |     |  locked  |     '--------'
      '--------'  |  |  mux M1  |--.  |  mux M2  |--.  .--------.
                  |  '----------'  |  '----------'  '--| dev D2 |
                  |  .--------.    |  .--------.       '--------'
                  '--| dev D4 |    '--| dev D3 |
                     '--------'       '--------'
  
  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.
  
  
  Mux-locked mux as parent of parent-locked mux
  ---------------------------------------------

  This is probably a bad topology::

  
                     .----------.     .----------.     .--------.
      .--------.     |   mux-   |-----|  parent- |-----| dev D1 |
      |  root  |--+--|  locked  |     |  locked  |     '--------'
      '--------'  |  |  mux M1  |--.  |  mux M2  |--.  .--------.
                  |  '----------'  |  '----------'  '--| dev D2 |
                  |  .--------.    |  .--------.       '--------'
                  '--| dev D4 |    '--| dev D3 |
                     '--------'       '--------'
  
  When device D1 is accessed, accesses to D2 and D3 are locked out
  for the full duration of the operation (M1 locks child muxes on the
  root adapter). But accesses to D4 are possibly interleaved at any
  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

  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

  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.

  
  But if M2 is not making the above stated assumption, and if M2 is not
  auto-closing, the topology is fine.
  
  
  Parent-locked mux as parent of mux-locked mux
  ---------------------------------------------

  This is a good topology::

  
                     .----------.     .----------.     .--------.
      .--------.     |  parent- |-----|   mux-   |-----| dev D1 |
      |  root  |--+--|  locked  |     |  locked  |     '--------'
      '--------'  |  |  mux M1  |--.  |  mux M2  |--.  .--------.
                  |  '----------'  |  '----------'  '--| dev D2 |
                  |  .--------.    |  .--------.       '--------'
                  '--| dev D4 |    '--| dev D3 |
                     '--------'       '--------'
  
  When 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). Accesses to D3 and D4 are possibly interleaved at
  any point, just as is expected for mux-locked muxes.
  
  When D3 or D4 are accessed, everything else is locked out. For D3
  accesses, M1 locks the root adapter. For D4 accesses, the root
  adapter is locked directly.
  
  
  Two mux-locked sibling muxes
  ----------------------------

  This is a good topology::

  
                                      .--------.
                     .----------.  .--| dev D1 |
                     |   mux-   |--'  '--------'
                  .--|  locked  |     .--------.
                  |  |  mux M1  |-----| dev D2 |
                  |  '----------'     '--------'
                  |  .----------.     .--------.
      .--------.  |  |   mux-   |-----| dev D3 |
      |  root  |--+--|  locked  |     '--------'
      '--------'  |  |  mux M2  |--.  .--------.
                  |  '----------'  '--| dev D4 |
                  |  .--------.       '--------'
                  '--| dev D5 |
                     '--------'
  
  When D1 is accessed, accesses to D2, D3 and D4 are locked out. But
  accesses to D5 may be interleaved at any time.
  
  
  Two parent-locked sibling muxes
  -------------------------------

  This is a good topology::

                                      .--------.

                     .----------.  .--| dev D1 |
                     |  parent- |--'  '--------'
                  .--|  locked  |     .--------.
                  |  |  mux M1  |-----| dev D2 |
                  |  '----------'     '--------'
                  |  .----------.     .--------.
      .--------.  |  |  parent- |-----| dev D3 |
      |  root  |--+--|  locked  |     '--------'
      '--------'  |  |  mux M2  |--.  .--------.
                  |  '----------'  '--| dev D4 |
                  |  .--------.       '--------'
                  '--| dev D5 |
                     '--------'
  
  When any device is accessed, accesses to all other devices are locked
  out.
  
  
  Mux-locked and parent-locked sibling muxes
  ------------------------------------------

  This is a good topology::

                                      .--------.

                     .----------.  .--| dev D1 |
                     |   mux-   |--'  '--------'
                  .--|  locked  |     .--------.
                  |  |  mux M1  |-----| dev D2 |
                  |  '----------'     '--------'
                  |  .----------.     .--------.
      .--------.  |  |  parent- |-----| dev D3 |
      |  root  |--+--|  locked  |     '--------'
      '--------'  |  |  mux M2  |--.  .--------.
                  |  '----------'  '--| dev D4 |
                  |  .--------.       '--------'
                  '--| dev D5 |
                     '--------'
  
  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.