Date: Mon, 17 Jun 2024 20:37:40 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>, 
	Robert Moore <robert.moore@...el.com>, linux-acpi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, acpica-devel@...ts.linux.dev
Subject: Re: [PATCH 1/5] ACPI: sysfs: convert utf-16 from _STR to utf-8 only once

On Thu, Jun 13, 2024 at 10:15 PM Thomas Weißschuh <linux@...ssschuh.net> wrote:
>
> The ACPI _STR method returns an UTF-16 string that is converted to utf-8
> before printing it in sysfs.
> Currently this conversion is performed every time the "description"
> sysfs attribute is read, which is not necessary.

Why is it a problem, though?

How many devices have _STR and how much of the time is it used?

Hint: On the system I'm sitting in front of, the answer is 0 and never.

So Is it really worth adding an _STR string pointer to every struct acpi_device?

> Only perform the conversion once and cache the result.
>
> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
>  drivers/acpi/device_sysfs.c | 63 ++++++++++++++++++++++++++++-----------------
>  include/acpi/acpi_bus.h     |  2 +-
>  2 files changed, 40 insertions(+), 25 deletions(-)

And it's more lines of code even.

>
> diff --git a/drivers/acpi/device_sysfs.c b/drivers/acpi/device_sysfs.c
> index 23373faa35ec..4bedbe8f57ed 100644
> --- a/drivers/acpi/device_sysfs.c
> +++ b/drivers/acpi/device_sysfs.c
> @@ -439,24 +439,11 @@ static ssize_t description_show(struct device *dev,
>                                 char *buf)
>  {
>         struct acpi_device *acpi_dev = to_acpi_device(dev);
> -       int result;
>
> -       if (acpi_dev->pnp.str_obj == NULL)
> +       if (acpi_dev->pnp.str == NULL)
>                 return 0;
>
> -       /*
> -        * The _STR object contains a Unicode identifier for a device.
> -        * We need to convert to utf-8 so it can be displayed.
> -        */
> -       result = utf16s_to_utf8s(
> -               (wchar_t *)acpi_dev->pnp.str_obj->buffer.pointer,
> -               acpi_dev->pnp.str_obj->buffer.length,
> -               UTF16_LITTLE_ENDIAN, buf,
> -               PAGE_SIZE - 1);
> -
> -       buf[result++] = '\n';
> -
> -       return result;
> +       return sysfs_emit("%s\n", acpi_dev->pnp.str);
>  }
>  static DEVICE_ATTR_RO(description);
>
> @@ -507,14 +494,46 @@ static ssize_t status_show(struct device *dev, struct device_attribute *attr,
>  }
>  static DEVICE_ATTR_RO(status);
>
> +static const char *acpi_device_str(struct acpi_device *dev)
> +{
> +       struct acpi_buffer buffer = {ACPI_ALLOCATE_BUFFER, NULL};
> +       union acpi_object *str_obj;
> +       acpi_status status;
> +       const char *ret;
> +       char buf[512];
> +       int result;
> +
> +       if (!acpi_has_method(dev->handle, "_STR"))
> +               return NULL;
> +
> +       status = acpi_evaluate_object(dev->handle, "_STR",
> +                                     NULL, &buffer);
> +       if (ACPI_FAILURE(status))
> +               return NULL;
> +
> +       str_obj = buffer.pointer;
> +       /*
> +        * The _STR object contains a Unicode identifier for a device.
> +        * We need to convert to utf-8 so it can be displayed.
> +        */
> +       result = utf16s_to_utf8s((wchar_t *)str_obj->buffer.pointer,
> +                                str_obj->buffer.length,
> +                                UTF16_LITTLE_ENDIAN,
> +                                buf, sizeof(buf) - 1);
> +       buf[result++] = '\0';
> +
> +       ret = kstrdup(buf, GFP_KERNEL);
> +       kfree(buffer.pointer);
> +
> +       return ret;
> +}
> +
>  /**
>   * acpi_device_setup_files - Create sysfs attributes of an ACPI device.
>   * @dev: ACPI device object.
>   */
>  int acpi_device_setup_files(struct acpi_device *dev)
>  {
> -       struct acpi_buffer buffer = {ACPI_ALLOCATE_BUFFER, NULL};
> -       acpi_status status;
>         int result = 0;
>
>         /*
> @@ -539,12 +558,8 @@ int acpi_device_setup_files(struct acpi_device *dev)
>         /*
>          * If device has _STR, 'description' file is created
>          */
> -       if (acpi_has_method(dev->handle, "_STR")) {
> -               status = acpi_evaluate_object(dev->handle, "_STR",
> -                                       NULL, &buffer);
> -               if (ACPI_FAILURE(status))
> -                       buffer.pointer = NULL;
> -               dev->pnp.str_obj = buffer.pointer;
> +       dev->pnp.str = acpi_device_str(dev);
> +       if (dev->pnp.str) {
>                 result = device_create_file(&dev->dev, &dev_attr_description);
>                 if (result)
>                         goto end;
> @@ -618,7 +633,7 @@ void acpi_device_remove_files(struct acpi_device *dev)
>          * If device has _STR, remove 'description' file
>          */
>         if (acpi_has_method(dev->handle, "_STR")) {
> -               kfree(dev->pnp.str_obj);
> +               kfree(dev->pnp.str);
>                 device_remove_file(&dev->dev, &dev_attr_description);
>         }
>         /*
> diff --git a/include/acpi/acpi_bus.h b/include/acpi/acpi_bus.h
> index 1a4dfd7a1c4a..32e3105c9ece 100644
> --- a/include/acpi/acpi_bus.h
> +++ b/include/acpi/acpi_bus.h
> @@ -254,7 +254,7 @@ struct acpi_device_pnp {
>         struct list_head ids;           /* _HID and _CIDs */
>         acpi_device_name device_name;   /* Driver-determined */
>         acpi_device_class device_class; /*        "          */
> -       union acpi_object *str_obj;     /* unicode string for _STR method */
> +       const char *str;                /* _STR */
>  };
>
>  #define acpi_device_bid(d)     ((d)->pnp.bus_id)
>
> --
> 2.45.2
>
>

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