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1================================2I2C muxes and complex topologies3================================4 5There are a couple of reasons for building more complex I2C topologies6than a straight-forward I2C bus with one adapter and one or more devices.7 8Some example use cases are:9 101. A mux may be needed on the bus to prevent address collisions.11 122. The bus may be accessible from some external bus master, and arbitration13 may be needed to determine if it is ok to access the bus.14 153. A device (particularly RF tuners) may want to avoid the digital noise16 from the I2C bus, at least most of the time, and sits behind a gate17 that has to be operated before the device can be accessed.18 19Several types of hardware components such as I2C muxes, I2C gates and I2C20arbitrators allow to handle such needs.21 22These components are represented as I2C adapter trees by Linux, where23each adapter has a parent adapter (except the root adapter) and zero or24more child adapters. The root adapter is the actual adapter that issues25I2C transfers, and all adapters with a parent are part of an "i2c-mux"26object (quoted, since it can also be an arbitrator or a gate).27 28Depending of the particular mux driver, something happens when there is29an I2C transfer on one of its child adapters. The mux driver can30obviously operate a mux, but it can also do arbitration with an external31bus master or open a gate. The mux driver has two operations for this,32select and deselect. select is called before the transfer and (the33optional) deselect is called after the transfer.34 35 36Locking37=======38 39There are two variants of locking available to I2C muxes, they can be40mux-locked or parent-locked muxes.41 42 43Mux-locked muxes44----------------45 46Mux-locked muxes does not lock the entire parent adapter during the47full select-transfer-deselect transaction, only the muxes on the parent48adapter are locked. Mux-locked muxes are mostly interesting if the49select and/or deselect operations must use I2C transfers to complete50their tasks. Since the parent adapter is not fully locked during the51full transaction, unrelated I2C transfers may interleave the different52stages of the transaction. This has the benefit that the mux driver53may be easier and cleaner to implement, but it has some caveats.54 55Mux-locked Example56~~~~~~~~~~~~~~~~~~57 58::59 60 .----------. .--------.61 .--------. | mux- |-----| dev D1 |62 | root |--+--| locked | '--------'63 '--------' | | mux M1 |--. .--------.64 | '----------' '--| dev D2 |65 | .--------. '--------'66 '--| dev D3 |67 '--------'68 69When there is an access to D1, this happens:70 71 1. Someone issues an I2C transfer to D1.72 2. M1 locks muxes on its parent (the root adapter in this case).73 3. M1 calls ->select to ready the mux.74 4. M1 (presumably) does some I2C transfers as part of its select.75 These transfers are normal I2C transfers that locks the parent76 adapter.77 5. M1 feeds the I2C transfer from step 1 to its parent adapter as a78 normal I2C transfer that locks the parent adapter.79 6. M1 calls ->deselect, if it has one.80 7. Same rules as in step 4, but for ->deselect.81 8. M1 unlocks muxes on its parent.82 83This means that accesses to D2 are lockout out for the full duration84of the entire operation. But accesses to D3 are possibly interleaved85at any point.86 87Mux-locked caveats88~~~~~~~~~~~~~~~~~~89 90When using a mux-locked mux, be aware of the following restrictions:91 92[ML1]93 If you build a topology with a mux-locked mux being the parent94 of a parent-locked mux, this might break the expectation from the95 parent-locked mux that the root adapter is locked during the96 transaction.97 98[ML2]99 It is not safe to build arbitrary topologies with two (or more)100 mux-locked muxes that are not siblings, when there are address101 collisions between the devices on the child adapters of these102 non-sibling muxes.103 104 I.e. the select-transfer-deselect transaction targeting e.g. device105 address 0x42 behind mux-one may be interleaved with a similar106 operation targeting device address 0x42 behind mux-two. The107 intent with such a topology would in this hypothetical example108 be that mux-one and mux-two should not be selected simultaneously,109 but mux-locked muxes do not guarantee that in all topologies.110 111[ML3]112 A mux-locked mux cannot be used by a driver for auto-closing113 gates/muxes, i.e. something that closes automatically after a given114 number (one, in most cases) of I2C transfers. Unrelated I2C transfers115 may creep in and close prematurely.116 117[ML4]118 If any non-I2C operation in the mux driver changes the I2C mux state,119 the driver has to lock the root adapter during that operation.120 Otherwise garbage may appear on the bus as seen from devices121 behind the mux, when an unrelated I2C transfer is in flight during122 the non-I2C mux-changing operation.123 124 125Parent-locked muxes126-------------------127 128Parent-locked muxes lock the parent adapter during the full select-129transfer-deselect transaction. The implication is that the mux driver130has to ensure that any and all I2C transfers through that parent131adapter during the transaction are unlocked I2C transfers (using e.g.132__i2c_transfer), or a deadlock will follow.133 134Parent-locked Example135~~~~~~~~~~~~~~~~~~~~~136 137::138 139 .----------. .--------.140 .--------. | parent- |-----| dev D1 |141 | root |--+--| locked | '--------'142 '--------' | | mux M1 |--. .--------.143 | '----------' '--| dev D2 |144 | .--------. '--------'145 '--| dev D3 |146 '--------'147 148When there is an access to D1, this happens:149 150 1. Someone issues an I2C transfer to D1.151 2. M1 locks muxes on its parent (the root adapter in this case).152 3. M1 locks its parent adapter.153 4. M1 calls ->select to ready the mux.154 5. If M1 does any I2C transfers (on this root adapter) as part of155 its select, those transfers must be unlocked I2C transfers so156 that they do not deadlock the root adapter.157 6. M1 feeds the I2C transfer from step 1 to the root adapter as an158 unlocked I2C transfer, so that it does not deadlock the parent159 adapter.160 7. M1 calls ->deselect, if it has one.161 8. Same rules as in step 5, but for ->deselect.162 9. M1 unlocks its parent adapter.163 10. M1 unlocks muxes on its parent.164 165This means that accesses to both D2 and D3 are locked out for the full166duration of the entire operation.167 168Parent-locked Caveats169~~~~~~~~~~~~~~~~~~~~~170 171When using a parent-locked mux, be aware of the following restrictions:172 173[PL1]174 If you build a topology with a parent-locked mux being the child175 of another mux, this might break a possible assumption from the176 child mux that the root adapter is unused between its select op177 and the actual transfer (e.g. if the child mux is auto-closing178 and the parent mux issues I2C transfers as part of its select).179 This is especially the case if the parent mux is mux-locked, but180 it may also happen if the parent mux is parent-locked.181 182[PL2]183 If select/deselect calls out to other subsystems such as gpio,184 pinctrl, regmap or iio, it is essential that any I2C transfers185 caused by these subsystems are unlocked. This can be convoluted to186 accomplish, maybe even impossible if an acceptably clean solution187 is sought.188 189 190Complex Examples191================192 193Parent-locked mux as parent of parent-locked mux194------------------------------------------------195 196This is a useful topology, but it can be bad::197 198 .----------. .----------. .--------.199 .--------. | parent- |-----| parent- |-----| dev D1 |200 | root |--+--| locked | | locked | '--------'201 '--------' | | mux M1 |--. | mux M2 |--. .--------.202 | '----------' | '----------' '--| dev D2 |203 | .--------. | .--------. '--------'204 '--| dev D4 | '--| dev D3 |205 '--------' '--------'206 207When any device is accessed, all other devices are locked out for208the full duration of the operation (both muxes lock their parent,209and specifically when M2 requests its parent to lock, M1 passes210the buck to the root adapter).211 212This topology is bad if M2 is an auto-closing mux and M1->select213issues any unlocked I2C transfers on the root adapter that may leak214through and be seen by the M2 adapter, thus closing M2 prematurely.215 216 217Mux-locked mux as parent of mux-locked mux218------------------------------------------219 220This is a good topology::221 222 .----------. .----------. .--------.223 .--------. | mux- |-----| mux- |-----| dev D1 |224 | root |--+--| locked | | locked | '--------'225 '--------' | | mux M1 |--. | mux M2 |--. .--------.226 | '----------' | '----------' '--| dev D2 |227 | .--------. | .--------. '--------'228 '--| dev D4 | '--| dev D3 |229 '--------' '--------'230 231When device D1 is accessed, accesses to D2 are locked out for the232full duration of the operation (muxes on the top child adapter of M1233are locked). But accesses to D3 and D4 are possibly interleaved at234any point.235 236Accesses to D3 locks out D1 and D2, but accesses to D4 are still possibly237interleaved.238 239 240Mux-locked mux as parent of parent-locked mux241---------------------------------------------242 243This is probably a bad topology::244 245 .----------. .----------. .--------.246 .--------. | mux- |-----| parent- |-----| dev D1 |247 | root |--+--| locked | | locked | '--------'248 '--------' | | mux M1 |--. | mux M2 |--. .--------.249 | '----------' | '----------' '--| dev D2 |250 | .--------. | .--------. '--------'251 '--| dev D4 | '--| dev D3 |252 '--------' '--------'253 254When device D1 is accessed, accesses to D2 and D3 are locked out255for the full duration of the operation (M1 locks child muxes on the256root adapter). But accesses to D4 are possibly interleaved at any257point.258 259This kind of topology is generally not suitable and should probably260be avoided. The reason is that M2 probably assumes that there will261be no I2C transfers during its calls to ->select and ->deselect, and262if there are, any such transfers might appear on the slave side of M2263as partial I2C transfers, i.e. garbage or worse. This might cause264device lockups and/or other problems.265 266The topology is especially troublesome if M2 is an auto-closing267mux. In that case, any interleaved accesses to D4 might close M2268prematurely, as might any I2C transfers part of M1->select.269 270But if M2 is not making the above stated assumption, and if M2 is not271auto-closing, the topology is fine.272 273 274Parent-locked mux as parent of mux-locked mux275---------------------------------------------276 277This is a good topology::278 279 .----------. .----------. .--------.280 .--------. | parent- |-----| mux- |-----| dev D1 |281 | root |--+--| locked | | locked | '--------'282 '--------' | | mux M1 |--. | mux M2 |--. .--------.283 | '----------' | '----------' '--| dev D2 |284 | .--------. | .--------. '--------'285 '--| dev D4 | '--| dev D3 |286 '--------' '--------'287 288When D1 is accessed, accesses to D2 are locked out for the full289duration of the operation (muxes on the top child adapter of M1290are locked). Accesses to D3 and D4 are possibly interleaved at291any point, just as is expected for mux-locked muxes.292 293When D3 or D4 are accessed, everything else is locked out. For D3294accesses, M1 locks the root adapter. For D4 accesses, the root295adapter is locked directly.296 297 298Two mux-locked sibling muxes299----------------------------300 301This is a good topology::302 303 .--------.304 .----------. .--| dev D1 |305 | mux- |--' '--------'306 .--| locked | .--------.307 | | mux M1 |-----| dev D2 |308 | '----------' '--------'309 | .----------. .--------.310 .--------. | | mux- |-----| dev D3 |311 | root |--+--| locked | '--------'312 '--------' | | mux M2 |--. .--------.313 | '----------' '--| dev D4 |314 | .--------. '--------'315 '--| dev D5 |316 '--------'317 318When D1 is accessed, accesses to D2, D3 and D4 are locked out. But319accesses to D5 may be interleaved at any time.320 321 322Two parent-locked sibling muxes323-------------------------------324 325This is a good topology::326 327 .--------.328 .----------. .--| dev D1 |329 | parent- |--' '--------'330 .--| locked | .--------.331 | | mux M1 |-----| dev D2 |332 | '----------' '--------'333 | .----------. .--------.334 .--------. | | parent- |-----| dev D3 |335 | root |--+--| locked | '--------'336 '--------' | | mux M2 |--. .--------.337 | '----------' '--| dev D4 |338 | .--------. '--------'339 '--| dev D5 |340 '--------'341 342When any device is accessed, accesses to all other devices are locked343out.344 345 346Mux-locked and parent-locked sibling muxes347------------------------------------------348 349This is a good topology::350 351 .--------.352 .----------. .--| dev D1 |353 | mux- |--' '--------'354 .--| locked | .--------.355 | | mux M1 |-----| dev D2 |356 | '----------' '--------'357 | .----------. .--------.358 .--------. | | parent- |-----| dev D3 |359 | root |--+--| locked | '--------'360 '--------' | | mux M2 |--. .--------.361 | '----------' '--| dev D4 |362 | .--------. '--------'363 '--| dev D5 |364 '--------'365 366When D1 or D2 are accessed, accesses to D3 and D4 are locked out while367accesses to D5 may interleave. When D3 or D4 are accessed, accesses to368all other devices are locked out.369 370 371Mux type of existing device drivers372===================================373 374Whether a device is mux-locked or parent-locked depends on its375implementation. The following list was correct at the time of writing:376 377In drivers/i2c/muxes/:378 379====================== =============================================380i2c-arb-gpio-challenge Parent-locked381i2c-mux-gpio Normally parent-locked, mux-locked iff382 all involved gpio pins are controlled by the383 same I2C root adapter that they mux.384i2c-mux-gpmux Normally parent-locked, mux-locked iff385 specified in device-tree.386i2c-mux-ltc4306 Mux-locked387i2c-mux-mlxcpld Parent-locked388i2c-mux-pca9541 Parent-locked389i2c-mux-pca954x Parent-locked390i2c-mux-pinctrl Normally parent-locked, mux-locked iff391 all involved pinctrl devices are controlled392 by the same I2C root adapter that they mux.393i2c-mux-reg Parent-locked394====================== =============================================395 396In drivers/iio/:397 398====================== =============================================399gyro/mpu3050 Mux-locked400imu/inv_mpu6050/ Mux-locked401====================== =============================================402 403In drivers/media/:404 405======================= =============================================406dvb-frontends/lgdt3306a Mux-locked407dvb-frontends/m88ds3103 Parent-locked408dvb-frontends/rtl2830 Parent-locked409dvb-frontends/rtl2832 Mux-locked410dvb-frontends/si2168 Mux-locked411usb/cx231xx/ Parent-locked412======================= =============================================413