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: <CAMuHMdVyxrqSdoR5VMfazwQWbXPmgMLxm-5XfxcJagv-kTuEHg@mail.gmail.com>
Date: Fri, 10 Jan 2025 11:04:37 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Adam Ford <aford173@...il.com>
Cc: Pei Xiao <xiaopei01@...inos.cn>, 
	Dominique Martinet <dominique.martinet@...ark-techno.com>, 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

Hi Adam,

On Thu, Jan 9, 2025 at 4:03 PM Adam Ford <aford173@...il.com> wrote:
> On Thu, Jan 9, 2025 at 2:45 AM Pei Xiao <xiaopei01@...inos.cn> wrote:
> > 在 2025/1/3 09:34, Pei Xiao 写道:
> > > 在 2025/1/2 23:04, Adam Ford 写道:
> > >> 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);
> > > do you mean:
> > > writeb(FIELD_PREP(REG12_CK_DIV_MASK, min(div, 3)), phy->regs + PHY_REG(12));
> >
> > Does anyone have any suggestions?
>
> Sorry for the delayed responses, I was traveling for the last week.
>
> What about replacing the for loop with a do-while loop where the
> criteria is int_pllclk < (50 * MHZ) && div <= 3.  Might that work?

Such a loop might never complete, although in practice that does not
seem to be possible (until someone modifies the tables in the driver
and adds a too-large cfg->pixclk?).

Probably the safest solution is to change the return type of
fsl_samsung_hdmi_phy_configure_pll_lock_det() from void to int, and
let it return an error code:

    if (div == 4)
            return -EINVAL;

 and propagate that. Its sole caller already returns an error code.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds