Re: [PATCH v2 0/8] i2c mux cleanup and locking update

[Peter Rosin]

Hi Antti,

On 2016-01-06 18:17, Antti Palosaari wrote:
> On 01/05/2016 05:57 PM, Peter Rosin wrote:
>> From: Peter Rosin <peda@xxxxxxxxxx>
>>
>> Hi!
>>
>> I have a pair of boards with this i2c topology:
>>
>>                         GPIO ---|  ------ BAT1
>>                          |      v /
>>     I2C  -----+------B---+---- MUX
>>               |                   \
>>             EEPROM                 ------ BAT2
>>
>>     (B denotes the boundary between the boards)
> 
> Handling of I2C muxes that close channel automatically, after the first I2C stop (P) is seen?
> 
> For example channel is selected to BAT1 => there is EEPROM write => mux closes channel BAT1 => access to BAT1 will fail.

The proposed locking changes affect gpio- and pinctrl-controlled muxes
only, and I can't see one of those actually know anything about the
i2c-signals that they mux. Such muxes certainly has to obey the pins
that control them, no?

That fact that other muxes might piggy-back and also use the new
i2c-controlled flag is a different question, and it may indeed not be
safe to declare a mux "i2c-controlled" just to avoid the locking, if
the locking is in fact required. Maybe the name "i2c-controlled" is
poor?

> Is it possible to lock whole adapter, but allow only traffic to i2c mux client?

This is basically what is there today, and which does not work for the
above i2c topology. So maybe you need to expand on what you meant?



There is no neat way for the i2c mux code to dip its fingers into all
relevant i2c-accesses to categorize them as "mux updates" or "mux slave
side accesses", when they come from generic subsystems such as gpio
och pinctrl.

The only thing I have been able to think of in that area is to add i2c
devices that are used to change the mux to a virtual slave adapter of
the mux, so that the mux can identify those accesses and bypass the
locking. But that isn't generic enough to cover the case where one
device is used to control more than one mux (since it needs to sit on
more than one adapter in that case), so I scrapped that idea pretty
early. But I also find that I have to scrap it again and again, since
everything else I cook up usually ends up being some variant of the
virtual slave adapter when I think some more about it. Maybe it *is*
possible to have the same device in different places in the adapter
tree (if all those places have a common ancestor)? But I don't think
so...

That does not address your concerns about extra accesses creeping
in between the mux select and the mux slave side access for auto-
closing muxes (or arbitrators), but I never pretended that there would
be protection from that. If there are such requirements you basically
have to lock the root adapter to prevent i2c "noise".

I can't see recursive locks helping either, because that would need
lock ownership to be propagated into gpio/pinctrl, and that is again
too ugly.

Cheers,
Peter

> regards
> Antti
> 
>>
>> The problem with this is that the GPIO controller sits on the same i2c bus
>> that it MUXes. For pca954x devices this is worked around by using unlocked
>> transfers when updating the MUX. I have no such luck as the GPIO is a general
>> purpose IO expander and the MUX is just a random bidirectional MUX, unaware
>> of the fact that it is muxing an i2c bus, and extending unlocked transfers
>> into the GPIO subsystem is too ugly to even think about. But the general hw
>> approach is sane in my opinion, with the number of connections between the
>> two boards minimized. To put is plainly, I need support for it.
>>
>> So, I observe that while it is needed to have the i2c bus locked during the
>> actual MUX update in order to avoid random garbage on the slave side, it
>> is not strictly a must to have it locked over the whole sequence of a full
>> select-transfer-deselect operation. The MUX itself needs to be locked, so
>> transfers to clients behind the mux are serialized, and the MUX needs to be
>> stable during all i2c traffic (otherwise individual mux slave segments
>> might see garbage).
>>
>> This series accomplishes this by adding a dt property to i2c-mux-gpio and
>> i2c-mux-pinctrl that can be used to state that the mux is updated by means
>> of the muxed master bus, and that the select-transfer-deselect operations
>> should be locked individually. When this holds, the i2c bus *is* locked
>> during muxing, since the muxing happens as part of i2c transfers. This
>> is true even if the MUX is updated with several transfers to the GPIO (at
>> least as long as *all* MUX changes are using the i2s master bus). A lock
>> is added to the mux so that transfers through the mux are serialized.
>>
>> Concerns:
>> - The locking is perhaps too complex?
>> - I worry about the priority inheritance aspect of the adapter lock. When
>>    the transfers behind the mux are divided into select-transfer-deselect all
>>    locked individually, low priority transfers get more chances to interfere
>>    with high priority transfers.
>> - When doing an i2c_transfer() in_atomic() context of with irqs_disabled(),
>>    there is a higher possibility that the mux is not returned to its idle
>>    state after a failed (-EAGAIN) transfer due to trylock.
>>
>> To summarize the series, there's some i2c-mux infrastructure cleanup work
>> first (I think that part stands by itself as desireable regardless), the
>> locking changes are in the last three patches of the series, with the real
>> meat in 8/8.
>>
>> PS. needs a bunch of testing, I do not have access to all the involved hw
>>
>> Changes since v1:
>> - Allocate mux core and (optional) priv in a combined allocation.
>> - Killed dev_err messages triggered by memory allocation failure.
>> - Fix the device specific i2c muxes that I had overlooked.
>> - Rebased on top of v4.4-rc8 (was based on v4.4-rc6 previously).
>>
>> Cheers,
>> Peter
>>
>> Peter Rosin (8):
>>    i2c-mux: add common core data for every mux instance
>>    i2c-mux: move select and deselect ops to i2c_mux_core
>>    i2c-mux: move the slave side adapter management to i2c_mux_core
>>    i2c-mux: remove the mux dev pointer from the mux per channel data
>>    i2c-mux: pinctrl: get rid of the driver private struct device pointer
>>    i2c: allow adapter drivers to override the adapter locking
>>    i2c: muxes always lock the parent adapter
>>    i2c-mux: relax locking of the top i2c adapter during i2c controlled
>>      muxing
>>
>>   .../devicetree/bindings/i2c/i2c-mux-gpio.txt       |   2 +
>>   .../devicetree/bindings/i2c/i2c-mux-pinctrl.txt    |   4 +
>>   drivers/i2c/i2c-core.c                             |  59 ++---
>>   drivers/i2c/i2c-mux.c                              | 272 +++++++++++++++++----
>>   drivers/i2c/muxes/i2c-arb-gpio-challenge.c         |  46 ++--
>>   drivers/i2c/muxes/i2c-mux-gpio.c                   |  58 ++---
>>   drivers/i2c/muxes/i2c-mux-pca9541.c                |  58 +++--
>>   drivers/i2c/muxes/i2c-mux-pca954x.c                |  66 ++---
>>   drivers/i2c/muxes/i2c-mux-pinctrl.c                |  89 +++----
>>   drivers/i2c/muxes/i2c-mux-reg.c                    |  63 ++---
>>   drivers/iio/imu/inv_mpu6050/inv_mpu_core.c         |  33 +--
>>   drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h          |   2 +-
>>   drivers/media/dvb-frontends/m88ds3103.c            |  23 +-
>>   drivers/media/dvb-frontends/m88ds3103_priv.h       |   2 +-
>>   drivers/media/dvb-frontends/rtl2830.c              |  24 +-
>>   drivers/media/dvb-frontends/rtl2830_priv.h         |   2 +-
>>   drivers/media/dvb-frontends/rtl2832.c              |  30 ++-
>>   drivers/media/dvb-frontends/rtl2832_priv.h         |   2 +-
>>   drivers/media/dvb-frontends/si2168.c               |  29 ++-
>>   drivers/media/dvb-frontends/si2168_priv.h          |   2 +-
>>   drivers/media/usb/cx231xx/cx231xx-core.c           |   6 +-
>>   drivers/media/usb/cx231xx/cx231xx-i2c.c            |  48 ++--
>>   drivers/media/usb/cx231xx/cx231xx.h                |   4 +-
>>   drivers/of/unittest.c                              |  41 ++--
>>   include/linux/i2c-mux-gpio.h                       |   2 +
>>   include/linux/i2c-mux-pinctrl.h                    |   2 +
>>   include/linux/i2c-mux.h                            |  39 ++-
>>   include/linux/i2c.h                                |  28 ++-
>>   28 files changed, 612 insertions(+), 424 deletions(-)
>>
> 
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