On 03/04/16 09:52, Peter Rosin wrote: > From: Peter Rosin <peda@xxxxxxxxxx> > > Signed-off-by: Peter Rosin <peda@xxxxxxxxxx> Very nice, one typo that I could see. > --- > Documentation/i2c/i2c-topology | 370 +++++++++++++++++++++++++++++++++++++++++ > MAINTAINERS | 1 + > 2 files changed, 371 insertions(+) > create mode 100644 Documentation/i2c/i2c-topology > > diff --git a/Documentation/i2c/i2c-topology b/Documentation/i2c/i2c-topology > new file mode 100644 > index 000000000000..7a10edd0874f > --- /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/ 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 especailly the case if the parent mux is mux-locked, but especially > + 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. > diff --git a/MAINTAINERS b/MAINTAINERS > index 03e00c7c88eb..d17afeb81246 100644 > --- a/MAINTAINERS > +++ b/MAINTAINERS > @@ -5274,6 +5274,7 @@ I2C MUXES > M: Peter Rosin <peda@xxxxxxxxxx> > L: linux-i2c@xxxxxxxxxxxxxxx > S: Maintained > +F: Documentation/i2c/i2c-topology > F: Documentation/i2c/muxes/ > F: Documentation/devicetree/bindings/i2c/i2c-mux* > F: drivers/i2c/i2c-mux.c > -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
