[RFC 0/4] TI camera serdes - I2C address translation draft

[Luca Ceresoli]

Hi,

there has been some discussion on linux-media about video
serializer/deserializer chipsets with remote I2C capabilities from TI
[0] and Maxim [1]. I took part discussing how the remote I2C feature
of such chips could be best implemented in Linux while I was
implementing a driver for the Texas Instruments DS90UB954-Q1 video
deserializer. My approach is different from both the one used by
Vladimir Zapolskiy on other TI chips, which look similar to the
DS90UB954 in their I2C management, and the one used by Kieran Bingham
with Maxim chips, which have a different and simpler I2C management.

After that I had to stop that work, so it is unfinished and I have no
plan to continue it. Upon suggestion by some linux-media developers
I'm sending my patches as RFC in the hope that they bring additional
material for the discussion.

I2C management is quite complete in my patches, and it shows how I
envisioned I2C management. For the rest the code is in large part
incomplete. Don't consider the V4L2, GPIO and other sections as ready
for any review.

The whole idea is structured around a central node, called the ATR
(Address Translator). It is similar to an I2C mux except it changes
the I2C addresses of transactions with an "alias" address for each
remote chip. Patch 2 has a detailed description of this process.


A typical setup looks like:

                          Slave X @ 0x10
                  .-----.   |
      .-----.     |     |---+---- B
      | CPU |--A--| ATR |
      `-----'     |     |---+---- C
                  `-----'   |
                          Slave Y @ 0x10

  A = "local" bus
  B = "remote" bus 0
  C = "remote" bus 1

In patch 2 I enriched the i2c-mux to also act as an ATR. However the
implementation grew larger than I desired, so now I think it would
make sense to leave i2c-mux as is, and add a new i2c-atr.c which has
ATR features without much of the MUX code. However the implementation
would not change too much, so you can look at i2c-mux to see how I
implemented the ATR.

In the ATR (i2c-mux.c) I implemented the logic needed to remap slave
addresses according to a table. Choosing appropriate aliases and
filling that table is driver-specific, so in this case it is done by
ds90ub954.c. The ATR driver needs to know when a new client appears on
the remote bus to setup translation and when it gets disconnected to
undo it. So I added a callback pair, attach_client and detach_client,
from i2c-core to i2c-mux and from there to the ATR driver. When
getting the callback the ATR driver chooses an alias to be used on the
local bus for the new chip, configures the ATR (perhaps setting some
registers) returns the alias back to the ATR which sill add the new
chip-alias pair to its table. The ATR (i2c-mux) then will do the
translation for each message, so that the alias will be used on the
local bus and the physical chip address on the remote bus.

The alias address for a new client is chosen from an alias pool that
must be defined in device tree. It is the responsibility of the DT
writer to fill the pool with addresses that are otherwise unused on
the local bus. The pool could not be filled automatically because
there might be conflicting chips on the local bus that are unknown to
the software, or that are just connected later.

The alias pool and the mapping done at runtime allow to model
different camera modules [or display or other modules] similarly to
beaglebone capes or rpi hats, up to a model where:

 1. there can be different camera modules being designed over time
 2. there can be different base boards being designed over time
 3. there is a standard interconnection between them (mechanical,
    electrical, communication bus)
 4. camera modules and base boards are designed and sold independently
    (thanks to point 3)

The implementation is split in the following patches:
 * Patch 1 adds the attach_client() and detach_client() callbacks to
   i2c-core
 * Patch 2 adds similar callbacks for the use of device drivers and,
   most importantly, implements the ATR engine
 * Patch 3 adds a farily complete DT bindings document, including the
   alias map
 * Patch 4 adds the DS90UB954-Q1 dual deserializer driver

There is no serializer driver here. The one I have is just a skeleton
setting a few registers, just enough to work on the deserializer
driver.

Each patch has an comprehensive list of open issues.

[0] https://www.spinics.net/lists/linux-gpio/msg33291.html
[1] https://www.spinics.net/lists/linux-media/msg142367.html

Regards,
--
Luca


Luca Ceresoli (4):
  i2c: core: let adapters be notified of client attach/detach
  i2c: mux: notify client attach/detach, add ATR
  media: dt-bindings: add DS90UB954-Q1 video deserializer
  media: ds90ub954: new driver for TI DS90UB954-Q1 video deserializer

 .../bindings/media/ti,ds90ub954-q1.txt        |  151 ++
 drivers/i2c/i2c-core-base.c                   |   16 +
 drivers/i2c/i2c-mux.c                         |  218 ++-
 drivers/i2c/muxes/i2c-mux-pca954x.c           |    2 +-
 drivers/media/Kconfig                         |    1 +
 drivers/media/Makefile                        |    2 +-
 drivers/media/serdes/Kconfig                  |   13 +
 drivers/media/serdes/Makefile                 |    1 +
 drivers/media/serdes/ds90ub954.c              | 1335 +++++++++++++++++
 include/linux/i2c-mux.h                       |   20 +-
 include/linux/i2c.h                           |    9 +
 11 files changed, 1760 insertions(+), 8 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/media/ti,ds90ub954-q1.txt
 create mode 100644 drivers/media/serdes/Kconfig
 create mode 100644 drivers/media/serdes/Makefile
 create mode 100644 drivers/media/serdes/ds90ub954.c

-- 
2.17.1