Re: [PATCH 2/3] vc04_services: mmal-vchiq: Use bool for vchiq_mmal_component.in_use

From: Dan Carpenter <error27@gmail.com>
To: Andrew Lunn <andrew@lunn.ch>
Cc: Umang Jain <umang.jain@ideasonboard.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Broadcom internal kernel review list
	<bcm-kernel-feedback-list@broadcom.com>,
	Ray Jui <rjui@broadcom.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Hans Verkuil <hverkuil-cisco@xs4all.nl>,
	Dave Stevenson <dave.stevenson@raspberrypi.com>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	linux-rpi-kernel@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	linux-staging@lists.linux.dev, linux-kernel@vger.kernel.org,
	kieran.bingham@ideasonboard.com
Subject: Re: [PATCH 2/3] vc04_services: mmal-vchiq: Use bool for vchiq_mmal_component.in_use
Date: Fri, 18 Nov 2022 12:01:34 +0300	[thread overview]
Message-ID: <Y3dJ7mETwKNVH7dU@kadam> (raw)
In-Reply-To: <Y3bQiDw11t4pRokP@lunn.ch>

On Fri, Nov 18, 2022 at 01:23:36AM +0100, Andrew Lunn wrote:
> >  struct vchiq_mmal_component {
> > -	u32 in_use:1;
> > +	bool in_use:1;
> >  	bool enabled:1;
> 
> The patch you referenced says:
> 
> +If a structure has many true/false values, consider consolidating them into a
> +bitfield with 1 bit members, or using an appropriate fixed width type, such as
> +u8.
> 
> The code did exactly this, using two bits fields, in one u32. A bool
> probably takes up 4 bytes, maybe 8 bytes, so this change probably
> doubles the storage size for these two fields.

In GCC and Clang bools take a byte, but the C language is vague and
other compilers are free to do it differently.

> Are these fields on the
> hot path, where an extra AND instruction would make a difference?

This patch takes the first u32 for "in_use" and squeezes it into the
same byte as "enabled" so it makes the struct four bytes smaller.  There
is still a 3 byte struct hole between "enabled" and "handle" so we could
add more 62 bool bitfields if we wanted.

In the v2 patch these become:

	bool in_use;
	bool enabled;

One byte each and there is a two byte gap before "handle".

regards,
dan carpenter