netdev - Re: [PATCH 1/3] can: rcar_canfd: Simplify clock handling

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Message-ID: <20240621-sensible-cerulean-pheasant-0a64e7-mkl@pengutronix.de>
Date: Fri, 21 Jun 2024 10:36:06 +0200
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Vincent MAILHOL <mailhol.vincent@...adoo.fr>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, linux-can@...r.kernel.org, linux-renesas-soc@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH 1/3] can: rcar_canfd: Simplify clock handling

On 06.06.2024 13:38:24, Geert Uytterhoeven wrote:
> > > > > @@ -545,8 +539,8 @@ struct rcar_canfd_global {
> > > > >         struct platform_device *pdev;   /* Respective platform device */
> > > > >         struct clk *clkp;               /* Peripheral clock */
> > > > >         struct clk *can_clk;            /* fCAN clock */
> > > > > -       enum rcar_canfd_fcanclk fcan;   /* CANFD or Ext clock */
> > > > >         unsigned long channels_mask;    /* Enabled channels mask */
> > > > > +       bool extclk;                    /* CANFD or Ext clock */
> > > > >         bool fdmode;                    /* CAN FD or Classical CAN only mode */
> > > >
> > > > Notwithstanding comment: you may consider to replace those two booleans by a:
> > > >
> > > >           unsigned int flags;
> > > >
> > > > This way, no more fields would be needed in the future if more quirks are added.
> > >
> > > Using "unsigned int flags" and BIT(x) flags would increase code size
> > > by 8 bytes (arm/arm64).
> >
> > I am not sure where you derive your figure from, but looking at the pahole:
> 
> pahole shows the size of data structures.
> 
> > > Using "unsigned int foo:1" bitfields would increase code size by 16
> > > (arm) or 12 (arm64) bytes.
> > > So as long as we can fit more bools inside the hole, it is more
> > > efficient to do so...
> >
> > I do not get this either. Where did you get your 16 bytes from? If I do:
> 
> I also looked at code size[*]: while storing bits takes less space than
> storing bytes, processing bits may require more instructions than
> processing bytes (depending on the architecture).
> 
> [*] size drivers/net/can/rcar/rcar_canfd.o

You have probably used "scripts/bloat-o-meter" from the kernel source
for this, right?

Marc

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