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: <CAHCN7xL8Yn4HQwgZAFgxbdVU6VSf6vGb2GEANeGRTaPKAA5uhw@mail.gmail.com>
Date: Thu, 9 Jan 2025 09:03:03 -0600
From: Adam Ford <aford173@...il.com>
To: Pei Xiao <xiaopei01@...inos.cn>
Cc: Dominique Martinet <dominique.martinet@...ark-techno.com>, 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 9, 2025 at 2:45 AM Pei Xiao <xiaopei01@...inos.cn> wrote:
>
>
> 在 2025/1/3 09:34, Pei Xiao 写道:
> > hi Adam,
> >
> > 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?

adam


>
> This compilation issue will result in errors on some versions of gcc,
>
> so it still needs to be resolved after all.
>
> >>> (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