linux-kernel - Re: [PATCH v2] media: v4l2-ctrl: Add p_def to v4l2_ctrl

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Date:   Mon, 21 Oct 2019 14:07:55 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Ricardo Ribalda Delgado <ribalda@...nel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] media: v4l2-ctrl: Add p_def to v4l2_ctrl_config

On 10/16/19 4:55 PM, Ricardo Ribalda Delgado wrote:
> This allows setting the default value on compound controls created via
> v4l2_ctrl_new_custom.
> 
> Signed-off-by: Ricardo Ribalda Delgado <ribalda@...nel.org>
> ---
>  drivers/media/v4l2-core/v4l2-ctrls.c | 3 ++-
>  include/media/v4l2-ctrls.h           | 2 ++
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
> index bf50d37ef6c1..939aa110daa0 100644
> --- a/drivers/media/v4l2-core/v4l2-ctrls.c
> +++ b/drivers/media/v4l2-core/v4l2-ctrls.c
> @@ -2583,7 +2583,8 @@ struct v4l2_ctrl *v4l2_ctrl_new_custom(struct v4l2_ctrl_handler *hdl,
>  			type, min, max,
>  			is_menu ? cfg->menu_skip_mask : step, def,
>  			cfg->dims, cfg->elem_size,
> -			flags, qmenu, qmenu_int, ptr_null, priv);
> +			flags, qmenu, qmenu_int,
> +			v4l2_ctrl_ptr_create((void *)cfg->p_def), priv);

I don't like this cast. I've been experimenting a bit and this works fine:

#include <stdlib.h>
#include <stdio.h>

union v4l2_ctrl_ptr {
        int *p_int;
        const int *p_int_const;
};

struct v4l2_ctrl_config {
        union v4l2_ctrl_ptr p;
};

static const int i = 5;

static const struct v4l2_ctrl_config c = {
        .p.p_int_const = &i,
};

int main(int argc, char **argv)
{
        printf("%d\n", *c.p.p_int_const);
        return 0;
}

So I would like to see a p_const being added to the union, and I think we
need a v4l2_ctrl_const_ptr_create() helper.

In a separate patch it would be really nice to replace any uses of .p with
.p_const where possible to signal that the contents isn't modified.

Regards,

	Hans

>  	if (ctrl)
>  		ctrl->is_private = cfg->is_private;
>  	return ctrl;
> diff --git a/include/media/v4l2-ctrls.h b/include/media/v4l2-ctrls.h
> index 26205ba3a0a0..d08d19a4ae34 100644
> --- a/include/media/v4l2-ctrls.h
> +++ b/include/media/v4l2-ctrls.h
> @@ -375,6 +375,7 @@ struct v4l2_ctrl_handler {
>   * @max:	The control's maximum value.
>   * @step:	The control's step value for non-menu controls.
>   * @def:	The control's default value.
> + * @p_def:	The control's default value for compound controls.
>   * @dims:	The size of each dimension.
>   * @elem_size:	The size in bytes of the control.
>   * @flags:	The control's flags.
> @@ -403,6 +404,7 @@ struct v4l2_ctrl_config {
>  	s64 max;
>  	u64 step;
>  	s64 def;
> +	const void *p_def;
>  	u32 dims[V4L2_CTRL_MAX_DIMS];
>  	u32 elem_size;
>  	u32 flags;
>