[PATCH] [media] staging: allow omap4iss to be modular

[Tony Lindgren]

* Laurent Pinchart <laurent.pinchart at ideasonboard.com> [140612 08:32]:
> Hi Tony,
> 
> On Thursday 12 June 2014 08:15:35 Tony Lindgren wrote:
> > * Laurent Pinchart <laurent.pinchart at ideasonboard.com> [140612 07:52]:
> > > On Wednesday 11 June 2014 07:47:54 Tony Lindgren wrote:
> > > > These should just use either pinctrl-single.c instead for muxing.
> > > > Or if they are not mux registers, we do have the syscon mapping
> > > > available in omap4.dtsi that pbias-regulator.c is already using.
> > > > 
> > > > Laurent, got any better ideas?
> > > 
> > > The ISS driver needs to write a single register, which contains several
> > > independent fields. They thus need to be controlled by a single driver.
> > > Some of them might be considered to be related to pinmuxing (although I
> > > disagree on that), others are certainly not about muxing (there are clock
> > > gate bits for instance).
> > > 
> > > Using the syscon mapping seems like the best option. I'll give it a try.
> > 
> > OK if it's not strictly pinctrl related then let's not use
> > pinctrl-single,bits for it. You may be able to implement one or more
> > framework drivers for it for pinctrl/regulator/clock/transceiver
> > whatever that register is doing.
> > 
> > In any case it's best to have that handling in a separate helper driver
> > somewhere as it's a separate piece of hardware from the camera module.
> > If it does not fit into any existing frameworks then it's best to have
> > it in a separate driver with the camera driver.
> 
> The register contains the following fields that control the two CSI2 PHYs 
> (PHY1 and PHY2).
> 
> 31    CAMERARX_CSI22_LANEENABLE2   PHY2 Lane 2 (CSI22_DX2, CSI22_DY2) Enable
> 30    CAMERARX_CSI22_LANEENABLE1   PHY2 Lane 1 (CSI22_DX1, CSI22_DY1) Enable
> 29    CAMERARX_CSI22_LANEENABLE0   PHY2 Lane 0 (CSI22_DX0, CSI22_DY0) Enable
> 28    CAMERARX_CSI21_LANEENABLE4   PHY1 Lane 4 (CSI21_DX4, CSI21_DY4) Enable
> 27    CAMERARX_CSI21_LANEENABLE3   PHY1 Lane 3 (CSI21_DX3, CSI21_DY3) Enable
> 26    CAMERARX_CSI21_LANEENABLE2   PHY1 Lane 2 (CSI21_DX2, CSI21_DY2) Enable
> 25    CAMERARX_CSI21_LANEENABLE1   PHY1 Lane 1 (CSI21_DX1, CSI21_DY1) Enable
> 24    CAMERARX_CSI21_LANEENABLE0   PHY1 Lane 0 (CSI21_DX0, CSI21_DY0) Enable
> 21    CAMERARX_CSI22_CTRLCLKEN     PHY2 Clock Enable
> 20:19 CAMERARX_CSI22_CAMMODE       PHY2 Mode (CCP2, CSI1, CSI2)
> 18    CAMERARX_CSI21_CTRLCLKEN     PHY1 Clock Enable
> 17:16 CAMERARX_CSI21_CAMMODE       PHY1 Mode (CCP2, CSI1, CSI2)
> 
> Bits 18 and 21 could be exposed through CCF. Bits 24 to 31 enable/disable the 
> CSI2 lanes, so it could be argued that they could be exposed through the 
> pinctrl framework. However, they need to be configured independently, possibly 
> at runtime. I'm thus not sure pinctrl would be a good idea. Bits 17:16 and 
> 20:19 don't fit in existing frameworks.

OK thanks for the info. Sounds like drivers/phy might be the right location
for it then and then the phy driver can use the syscon regmap.
 
> Given that this register is specific to the ISS, I think handling it as a 
> separate device through a separate driver would only complicate the 
> implementation without any real benefit.

Even though it's one register, it shoud still be treated separately from
the camera driver. The problems with keeping the register access to the
control module in the camera driver are at least following:

1. They live in separate hardware modules that can be clocked separately

2. Doing a read-back to flush a posted write in one hardware module most
   likely won't flush the write to other and that can lead into hard to
   find mysterious bugs

3. If we ever have a common system control module driver, we need to
   rewrite all the system control module register tinkering in the drivers

So it's best to try to use an existing framework for it. That avoids
tons of pain later on ;)

Regards,

Tony