linux-kernel - Re: [PATCH v6 11/15] software node: move small properties inline when copying

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Date:   Wed, 06 Nov 2019 00:42:02 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v6 11/15] software node: move small properties inline when copying

On Wednesday, October 23, 2019 10:02:29 PM CET Dmitry Torokhov wrote:
> When copying/duplicating set of properties, move smaller properties that
> were stored separately directly inside property entry structures. We can
> move:
> 
> - up to 8 bytes from U8 arrays
> - up to 4 words
> - up to 2 double words
> - one U64 value
> - one or 2 strings.

Yes, we can do that, but how much of a difference does this really make?

Also, how can one distinguish between a single-value property and an inline
array which this change?  By looking at the length?

> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> ---
>  drivers/base/swnode.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/base/swnode.c b/drivers/base/swnode.c
> index 18a30fb3cc58..49e1108aa4b7 100644
> --- a/drivers/base/swnode.c
> +++ b/drivers/base/swnode.c
> @@ -280,6 +280,16 @@ static int property_entry_copy_data(struct property_entry *dst,
>  	if (!dst->name)
>  		goto out_free_data;
>  
> +	if (!dst->is_inline && dst->length <= sizeof(dst->value)) {
> +		/* We have an opportunity to move the data inline */
> +		const void *tmp = dst->pointer;
> +
> +		memcpy(&dst->value, tmp, dst->length);
> +		dst->is_inline = true;
> +
> +		kfree(tmp);

This would have been more useful if we had been able to avoid making the
allocation altogether.

> +	}
> +
>  	return 0;
>  
>  out_free_data:
>