i2c: mux: document i2c muxes and elaborate on parent-/mux-locked muxes

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/i2c/i2c-topology

+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.
+
+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-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/
+imu/inv_mpu6050/          Parent-locked
+
+In drivers/media/
+dvb-frontends/m88ds3103   Parent-locked
+dvb-frontends/rtl2830     Parent-locked
+dvb-frontends/rtl2832     Parent-locked
+dvb-frontends/si2168      Parent-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 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
+     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.

MAINTAINERS

@@ -5275,6 +5275,7 @@ I2C MUXES
 M:	Peter Rosin <peda@axentia.se>
 L:	linux-i2c@vger.kernel.org
 S:	Maintained
+F:	Documentation/i2c/i2c-topology
 F:	Documentation/i2c/muxes/
 F:	Documentation/devicetree/bindings/i2c/i2c-mux*
 F:	drivers/i2c/i2c-mux.c