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

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Date:   Fri, 18 Nov 2022 12:01:34 +0300
From:   Dan Carpenter <error27@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Umang Jain <umang.jain@...asonboard.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        Ray Jui <rjui@...adcom.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kieran.bingham@...asonboard.com
Subject: Re: [PATCH 2/3] vc04_services: mmal-vchiq: Use bool for
 vchiq_mmal_component.in_use

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