linux-kernel - Re: [PATCH v9 04/20] gpio: uapi: define uAPI v2

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Date:   Tue, 22 Sep 2020 09:41:13 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Kent Gibson <warthog618@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH v9 04/20] gpio: uapi: define uAPI v2

On Tue, Sep 22, 2020 at 4:34 AM Kent Gibson <warthog618@...il.com> wrote:
> +/**
> + * struct gpio_v2_line_attribute - a configurable attribute of a line
> + * @id: attribute identifier with value from &enum gpio_v2_line_attr_id
> + * @padding: reserved for future use and must be zero filled
> + * @flags: if id is GPIO_V2_LINE_ATTR_ID_FLAGS, the flags for the GPIO
> + * line, with values from enum gpio_v2_line_flag, such as
> + * GPIO_V2_LINE_FLAG_ACTIVE_LOW, GPIO_V2_LINE_FLAG_OUTPUT etc, OR:ed
> + * together.  This overrides the default flags contained in the &struct
> + * gpio_v2_line_config for the associated line.
> + * @values: if id is GPIO_V2_LINE_ATTR_ID_OUTPUT_VALUES, a bitmap
> + * containing the values to which the lines will be set, with each bit
> + * number corresponding to the index into &struct
> + * gpio_v2_line_request.offsets.
> + * @debounce_period_us: if id is GPIO_V2_LINE_ATTR_ID_DEBOUNCE, the desired
> + * debounce period, in microseconds
> + */
> +struct gpio_v2_line_attribute {
> +       __u32 id;
> +       __u32 padding;
> +       union {
> +               __aligned_u64 flags;
> +               __aligned_u64 values;
> +               __u32 debounce_period_us;
> +       };
> +};

Having different-sized members in the union makes it hard for
something like strace to print the contents. How about just making
them all __aligned_u64 even when 32 bits are sufficient?

> +struct gpio_v2_line_request {
> +       __u32 offsets[GPIO_V2_LINES_MAX];
> +       char consumer[GPIO_MAX_NAME_SIZE];
> +       struct gpio_v2_line_config config;
> +       __u32 num_lines;
> +       __u32 event_buffer_size;
> +       /* Pad to fill implicit padding and reserve space for future use. */
> +       __u32 padding[5];
> +       __s32 fd;
> +};

> +struct gpio_v2_line_info {
> +       char name[GPIO_MAX_NAME_SIZE];
> +       char consumer[GPIO_MAX_NAME_SIZE];
> +       __u32 offset;
> +       __u32 num_attrs;
> +       __aligned_u64 flags;
> +       struct gpio_v2_line_attribute attrs[GPIO_V2_LINE_NUM_ATTRS_MAX];
> +       /* Space reserved for future use. */
> +       __u32 padding[4];
> +};

These are both several hundred bytes long, requiring a lot of data
to be copied to the stack and take up space there. I see this is not
actually much different for the v1 API, but I wonder if there has been
any analysis of whether this has a noticeable effect on application
runtime.

         Arnd