Date:	Thu, 14 Aug 2014 10:25:28 +0200
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Sakari Ailus <sakari.ailus@....fi>
Cc:	linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	kyungmin.park@...sung.com, b.zolnierkie@...sung.com,
	Hans Verkuil <hans.verkuil@...co.com>
Subject: Re: [PATCH/RFC v4 15/21] media: Add registration helpers for V4L2 flash

On 08/14/2014 06:34 AM, Sakari Ailus wrote:
> Hi Jacek,
>
> On Mon, Aug 11, 2014 at 03:27:22PM +0200, Jacek Anaszewski wrote:
>
> ...
>
>>>>>> diff --git a/include/media/v4l2-flash.h b/include/media/v4l2-flash.h
>>>>>> new file mode 100644
>>>>>> index 0000000..effa46b
>>>>>> --- /dev/null
>>>>>> +++ b/include/media/v4l2-flash.h
>>>>>> @@ -0,0 +1,137 @@
>>>>>> +/*
>>>>>> + * V4L2 Flash LED sub-device registration helpers.
>>>>>> + *
>>>>>> + *	Copyright (C) 2014 Samsung Electronics Co., Ltd
>>>>>> + *	Author: Jacek Anaszewski <j.anaszewski@...sung.com>
>>>>>> + *
>>>>>> + * This program is free software; you can redistribute it and/or modify
>>>>>> + * it under the terms of the GNU General Public License version 2 as
>>>>>> + * published by the Free Software Foundation."
>>>>>> + */
>>>>>> +
>>>>>> +#ifndef _V4L2_FLASH_H
>>>>>> +#define _V4L2_FLASH_H
>>>>>> +
>>>>>> +#include <media/v4l2-ctrls.h>
>>>>>> +#include <media/v4l2-device.h>
>>>>>> +#include <media/v4l2-dev.h>le
>>>>>> +#include <media/v4l2-event.h>
>>>>>> +#include <media/v4l2-ioctl.h>
>>>>>> +
>>>>>> +struct led_classdev_flash;
>>>>>> +struct led_classdev;
>>>>>> +enum led_brightness;
>>>>>> +
>>>>>> +struct v4l2_flash_ops {
>>>>>> +	int (*torch_brightness_set)(struct led_classdev *led_cdev,
>>>>>> +					enum led_brightness brightness);
>>>>>> +	int (*torch_brightness_update)(struct led_classdev *led_cdev);
>>>>>> +	int (*flash_brightness_set)(struct led_classdev_flash *flash,
>>>>>> +					u32 brightness);
>>>>>> +	int (*flash_brightness_update)(struct led_classdev_flash *flash);
>>>>>> +	int (*strobe_set)(struct led_classdev_flash *flash, bool state);
>>>>>> +	int (*strobe_get)(struct led_classdev_flash *flash, bool *state);
>>>>>> +	int (*timeout_set)(struct led_classdev_flash *flash, u32 timeout);
>>>>>> +	int (*indicator_brightness_set)(struct led_classdev_flash *flash,
>>>>>> +					u32 brightness);
>>>>>> +	int (*indicator_brightness_update)(struct led_classdev_flash *flash);
>>>>>> +	int (*external_strobe_set)(struct led_classdev_flash *flash,
>>>>>> +					bool enable);
>>>>>> +	int (*fault_get)(struct led_classdev_flash *flash, u32 *fault);
>>>>>> +	void (*sysfs_lock)(struct led_classdev *led_cdev);
>>>>>> +	void (*sysfs_unlock)(struct led_classdev *led_cdev);
>>>>>
>>>>> These functions are not driver specific and there's going to be just one
>>>>> implementation (I suppose). Could you refresh my memory regarding why
>>>>> the LED framework functions aren't called directly?
>>>>
>>>> These ops are required to make possible building led-class-flash as
>>>> a kernel module.
>>>
>>> Assuming you'd use the actual implementation directly, what would be the
>>> dependencies? I don't think the LED flash framework has any callbacks
>>> towards the V4L2 (LED) flash framework, does it? Please correct my
>>> understanding if I'm missing something. In Makefile format, assume all
>>> targets are .PHONY:
>>>
>>> led-flash-api: led-api
>>>
>>> v4l2-flash: led-flash-api
>>>
>>> driver: led-flash-api v4l2-flash
>>
>> LED Class Flash driver gains V4L2 Flash API when
>> CONFIG_V4L2_FLASH_LED_CLASS is defined. This is accomplished in
>> the probe function by either calling v4l2_flash_init function
>> or the macro of this name, when the CONFIG_V4L2_FLASH_LED_CLASS
>> macro isn't defined.
>>
>> If the v4l2-flash.c was to call the LED API directly, then the
>> led-class-flash module symbols would have to be available at
>> v4l2-flash.o linking time.
>
> Is this an issue? EXPORT_SYMBOL_GPL() for the relevant symbols should be
> enough.

It isn't enough. If I call e.g. led_set_flash_brightness
directly from v4l2-flash.c and configure led-class-flash to be built as
a module then I am getting "undefined reference to
led_set_flash_brightness" error during linking phase.

It happens because the linker doesn't take into account
led-flash-class.ko symbols. It is reasonable because initially
the kernel boots up without led-flash-class.ko module and
the processor wouldn't know the address to jump to in the
result of calling a led API function.
The led-class-flash.ko binary code is loaded into memory not
sooner than after executing "insmod led-class-flash.ko".

After linking dynamically with kernel the LED API function
addresses are relocated, and the LED Flash Class core can
initialize the v4l2_flash_ops structure. Then every LED Flash Class
driver can obtain the address of this structure with
led_get_v4l2_flash_ops and pass it to the v4l2_flash_init.

>> This requirement cannot be met if the led-class-flash is built
>> as a module.
>>
>> Use of function pointers in the v4l2-flash.c allows to compile it
>> into the kernel and enables the possibility of adding the V4L2 Flash
>> support conditionally, during driver probing.
>
> I'd simply decide this during kernel compilation time. If you want
> something, just enable it. v4l2_flash_init() is called directly by the
> driver in any case, so unless that is also called through a wrapper the
> driver is still directly dependent on it.

The problem is that v4l2-flash.o would have to depend on
led-class-flash.o, which when built as a module isn't available
during v4l2-flash.o linking time. In order to avoid v4l2-flash.o linking
problem, it would have to be built as a module.

Nevertheless, in this arrangement, the CONFIG_V4L2_FLASH_LED_CLASS
macro would be defined only in v4l2-flash.ko module, and
a LED Flash Class driver couldn't check whether V4L2 Flash support
is enabled. Its dependence on v4l2-flash.o would have to be fixed,
which is not what we want.

I have tested all these cases.

Best Regards,
Jacek Anaszewski
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