linux-kernel - Re: [PATCH] phy: freescale: fsl-samsung-hdmi: fix build error in fsl_samsung_hdmi_phy_configure_pll_lock

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Message-ID: <CAHCN7x+XNes4DbW1OvTjwsCddw5FJesCbPkv88rfRHM6sGBKSg@mail.gmail.com>
Date: Thu, 2 Jan 2025 09:04:00 -0600
From: Adam Ford <aford173@...il.com>
To: Dominique Martinet <dominique.martinet@...ark-techno.com>
Cc: Pei Xiao <xiaopei01@...inos.cn>, geert@...ux-m68k.org, aford@...conembedded.com, 
	arnd@...db.de, frieder.schrempf@...tron.de, kishon@...nel.org, 
	l.stach@...gutronix.de, linux-kernel@...r.kernel.org, 
	linux-phy@...ts.infradead.org, m.felsch@...gutronix.de, sandor.yu@....com, 
	u.kleine-koenig@...libre.com, vkoul@...nel.org
Subject: Re: [PATCH] phy: freescale: fsl-samsung-hdmi: fix build error in fsl_samsung_hdmi_phy_configure_pll_lock_det

On Thu, Jan 2, 2025 at 6:15 AM Dominique Martinet
<dominique.martinet@...ark-techno.com> wrote:
>
> Adam Ford wrote on Mon, Dec 30, 2024 at 08:11:16PM -0600:
> > > index 5eac70a1e858..3e4d1a5160ea 100644
> > > --- a/drivers/phy/freescale/phy-fsl-samsung-hdmi.c
> > > +++ b/drivers/phy/freescale/phy-fsl-samsung-hdmi.c
> > > @@ -341,7 +341,7 @@ fsl_samsung_hdmi_phy_configure_pll_lock_det(struct fsl_samsung_hdmi_phy *phy,
> > >                         break;
> > >         }
> > >
> > > -       writeb(FIELD_PREP(REG12_CK_DIV_MASK, div), phy->regs + PHY_REG(12));
> > > +       writeb(FIELD_PREP(REG12_CK_DIV_MASK, div == 4 ? div - 1 : div), phy->regs + PHY_REG(12));
> >
> > The for-loop above this line states:   for (div = 0; div < 4; div++)
> > How could this ever reach 4?  If it did reach 4, the calculation for
> > int_pllclk would need to be recalculated since int_pllclk = pclk / (1
> > << div);
>
> But... for (div = 0; div < 4; div++) does reach 4, if the break
> condition didn't match, which is something the compiler cannot ensure
> here.
>
> The old code would just fall out of any of the switch cases and fallback
> to div = 1 if pixclk > 297000000, which is likely incorrect, so in that
> sense just padding this through `& 3` and pretending it will never
> happen is probably acceptable, but this ought to have a better comment
> than what Pei just sent.

Maybe we use the MAX function to set div = max(div,3);

> (this was correct with the old lookup tables, I'm not sure if we can't
> compute any higher frequencies now?)
>
> My preference would be to actually check and handle this somehow since I
> don't think this part of the code is that performance critical that we
> can't afford an extra instruction, e.g. something like that:
> if (WARN_ONCE(div == 4, "pixclk %u out of range", pclk))
>         (appropriate fallback or return?)
>
> but I haven't spent the time to actually check so will leave that up to
> you.
>
> Thank you both,
> --
> Dominique