Re: [PATCH 0/3] capebus moving omap_devices to mach-omap2

[Pantelis Antoniou]

Hi Felibe,

On Nov 1, 2012, at 12:14 AM, Felipe Balbi wrote:

> Hi,
> 
> On Wed, Oct 31, 2012 at 11:36:25PM +0200, Pantelis Antoniou wrote:
>>> * Pantelis Antoniou <panto@xxxxxxxxxxxxxxxxxxxxxxx> [121031 13:14]:
>>>> On Oct 31, 2012, at 9:55 PM, Benoit Cousson wrote:
>>>>> 
>>>>> Yeah, I do agree. I'm confused as well. Only OMAP IPs under PRCM control
>>>>> could have an hwmod and thus must be handled by an omap_device.
>>>>> 
>>>>> Any devices that is created later cannot be omap_device. The DT core
>>>>> will create regular platform_device for them.
>>>>> 
>>>>> Since cape is an external board, it should have nothing to do with
>>>>> omap_device.
>>>>> 
>>>>> Looking at your patch:
>>>>> da8xx-dt: Create da8xx DT adapter device
>>>>> 
>>>>> I understand why you do that, but in fact that patch does not make sense
>>>>> to me :-(
>>>>> 
>>>>> Why do you have to create an omap_device from the driver probe?
>>>>> 
>>>> 
>>>> The problem is that capes are not external boards in the normal sense
>>>> as a PCI board is. In the PCI case the hardware that implements the 
>>>> desired functionality is on the PCI board, while in the cape case the
>>>> hardware module is in the SoC. The cape most of the times is quite
>>>> simple and contains passive components, leds or some kind of I2C/SPI devices.
>>> 
>>> Ah now I see, you're talking about the beaglebone extension
>>> boards :)
>>> 
>>> The way to deal with those properly is to just edit the
>>> board .dts entry to include omap-beaglebone-cape-xyz.dtsi
>>> whatever.
>>> 
>> 
>> That is what is being done...
>> This is the fallout.
>> 
>>>> You can't instantiate the omap_device early in the boot process like it was done up to 
>>>> now in the board file. You can only do that later in the boot process (for built-in 
>>>> cape drivers), or even after user-space has booted and the matching cape driver module
>>>> has been loaded.
>>> 
>>> Yes you can, just edit the .dts file for the extension
>>> board you have connected. This stuff should be DT
>>> only for sure, let's not even start adding platform_data
>>> entries for that.
>>> 
>>> The omap_device entries for the omap internal devices
>>> will be created automatically during startup based on the
>>> .dts. Note that you can set status = "disabled" for the
>>> omap internal devices in the .dts file, the devices are
>>> there in the hardware.
>>> 
>>> If you are sure the extension boards are safely hot
>>> pluggable, then all you need is some interface to enable
>>> and disable devices after booting. But that should be
>>> done in Linux generic way.
>>> 
>>> So we should not export any omap_hwmod or omap_device
>>> things, and want to keep it __init only, and local to
>>> arch/arm/mach-omap2.
>>> 
>> 
>> I disagree. This is not something that is handled by just
>> editing the dts. The way it is handled, is in the Linux
>> generic way, we have a proper bus, and the drivers as compiled
>> as standalone file. 
> 
> when you have an actual bus, that'd be correct.

What do you think capebus is? It is an actual bus that allows you
to do so.

> 
>> We're hitting a use case that wasn't there in omap until now.
>> 
>> There a a whole bunch of conflicting capes. There's no
>> way to instantiate them together. They must be instantiated
>> only after their EEPROMs are read and they are matched
>> to their corresponding cape drivers.
> 
> why this requirement of instantiating them only after reading EEPROMs ?
> 
> It's unnecessary to add that requirement, just do what Tony said
> (include my-awesome-cape.dtsi to bone.dts) and all i2c/spi devices
> should be created automatically.
> 
> The thing is that there is no such thing as a cape-device. The cape is
> just a collection of I2C, 1-wire and SPI devices anyway. What we should
> instantiate is bmp085, tls2550, sht21, instead of instantiating
> beagle-bone-weather-cape.
> 
> What's the problem in just instantiating all of those from bone.dts ?
> Expose the EEPROMs to userland so whatever SW you guys have running on
> the bone can figure out what features to expose to the SDK which the
> user sees, but the kernel doesn't need to know that there is a
> weather-cape attached to the bone.
> 

The I2C/SPI devices are not a problem at all.

Let me try to explain what the problem is, and why it all this 
is needed.

First of all, capes may be relatively simple boards, which may function
as a generic device - like the generic capes. They might not necessarily 
be simple. Some capes, for example the geiger cape have a number of
components that perform a specific task; in this instance the driving
circuit of the geiger tube and the event detector input. Other capes
that are being designed now are even more complex, i.e. there's a 
radar cape, an fpga cape and so on. So for these kind of boards
you will need a specific driver. That driver will probably use some 
generic-cape bits (like gpios, pwms, keys what-ever).
But it will put them to use in the custom in-kernel driver in it's own
way. You can't put that task to user-space, if it's ever slightly complex.  

So for really simple capes, after considerable pain you might, just
might, dump the problem to user-space and try to make it work. 
People have tried that and it's a total pain. There is no way that this will
work with anything more complex than a generic cape.

Then there's the out of the box experience problem.

What a customer that buys one of these boards along with a number of
capes wants is to plug them, and have them work. You cannot have
the DTS file activating all the drivers for each possible cape since
they conflict; the pins that one SPI bus that one cape uses might be a 
PWM output for another, and so on. 

Asking the customer to edit the distro supplied kernel's dts and recompiling,
while figuring out how to solve conflicts is not going to fly.

We have the versioning problem. All the standard capes go through
a very fast evolution processes. So we now have 3 versions of a DVI cape,
2 of a small LCD cape and so on. We don't want to abandon the capes that
are already sold and are out on the field, but we don't want to spend
too much time hacking the driver to support all the different version.
The version capability of capebus handles that; taking as an example the
DVI cape in which the power down GPIO moved around:

>        version@00A0 {
>                 version = "00A0";
>                 dvi {
>                         compatible = "da8xx-dt";
>                         pinctrl-names = "default";
>                         pinctrl-0 = <&bone_dvi_cape_dvi_00A0_pins>;
>                         ti,hwmods = "lcdc";
> 
>                         disp-pll = <560000000>;
>                         panel-type = "1024x768@60";
>                         powerdn-gpio = <&gpio2 7 0>;
>                 };
>         };
> 
>         version@00A1 {
>                 version = "00A1", "01";
>                 dvi {
>                         compatible = "da8xx-dt";
>                         pinctrl-names = "default";
>                         pinctrl-0 = <&bone_dvi_cape_dvi_00A1_pins>;
>                         ti,hwmods = "lcdc";
> 
>                         disp-pll = <560000000>;
>                         panel-type = "1024x768@60";
>                         powerdn-gpio = <&gpio2 31 0>;
>                 };
>         };
> 
> 

Finally, we have the target user-base problem. The users are all going to
be hobbyists, persons that buy beaglebones as a step up from Arduino. They
will want to hack their own kind of hardware; they might not even have
an EEPROM in their custom boards. They need some kind of runtime selection
of the cape they have defined, these is handled by the case of overrides.

For example, the Adafruit 1.8 Display is pretty popular, but it's not a cape.
You have to wire it yourself and connect it to the expansion header. Then
there's two different kinds of the display. It also has a PWM input for 
backlight control.

Making use of it is simple with capebus:

# cd /sys/devices/capebus.10
# echo "0:Adafruit 1.8 Cape" >slots

And that's it. The disable spi & pwm device the display needs will
be turned on, the pinmux it needs will be set, and the spi device
that the display uses will be created.

I have no clue how this can be simpler using any kind of user-space
method. And no, you can't have this active in the DTS of the common
kernel for all beaglebones.

> &bone_adafruit_cape {   
>         board-name = "Adafruit 1.8 Cape";
>                         
>         backlight {
>                 compatible      = "pwm-backlight";
>                 pinctrl-names   = "default";
>                 pinctrl-0       = <&pwm_bl_pins>;
> 
>                 pwms = <&ehrpwm1 1 500000 0>;
>                 pwm-names = "st7735fb";
>                 brightness-levels = <0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
>                 default-brightness-level = <50>; /* index to the array above */
>         };
>         
>         spi1-devices {
>                 compatible = "spi-dt";
> 
>                 #address-cells = <1>;
>                 #size-cells = <0>;
>                 
>                 parent = <&spi1>;
>         
>                 lcd@0 { 
>                         compatible = "adafruit,tft-lcd-1.8-red", "sitronix,st7735";
>                         spi-max-frequency = <8000000>;
>                         reg = <0>;
>                         spi-cpol;
>                         spi-cpha;
>                         pinctrl-names = "default";
>                         pinctrl-0 = <&lcd_pins>;
>                         st7735-rst = <&gpio4 19 0>;
>                         st7735-dc = <&gpio4 21 0>;
>                 };      
>         
>         };
> };      
> 


> -- 
> balbi

Regards

-- Pantelis

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