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diff --git a/Documentation/i2c/i2c-topology b/Documentation/i2c/i2c-topology new file mode 100644 index 000000000..e0aefeece --- /dev/null +++ b/Documentation/i2c/i2c-topology @@ -0,0 +1,370 @@ +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/ Mux-locked + +In drivers/media/ +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 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. |