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Date:   Thu, 6 Apr 2023 15:17:45 +0200
From:   Thierry Reding <thierry.reding@gmail.com>
To:     Conor Dooley <conor@kernel.org>
Cc:     Conor Dooley <conor.dooley@microchip.com>,
        Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        Daire McNamara <daire.mcnamara@microchip.com>,
        linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org,
        linux-riscv@lists.infradead.org
Subject: Re: [PATCH v15 1/2] pwm: add microchip soft ip corePWM driver
Message-ID: <ZC7GeXJbB9PAF0lb@orome>
References: <20230330071203.286972-1-conor.dooley@microchip.com>
 <20230330071203.286972-2-conor.dooley@microchip.com>
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On Sat, Apr 01, 2023 at 09:50:47PM +0100, Conor Dooley wrote:
> On Thu, Mar 30, 2023 at 08:12:03AM +0100, Conor Dooley wrote:
>=20
> > +	/*
> > +	 * Because 0xff is not a permitted value some error will seep into the
> > +	 * calculation of prescale as prescale grows. Specifically, this error
> > +	 * occurs where the remainder of the prescale calculation is less than
> > +	 * prescale.
> > +	 * For small values of prescale, only a handful of values will need
> > +	 * correction, but overall this applies to almost half of the valid
> > +	 * values for tmp.
> > +	 *
> > +	 * To keep the algorithm's decision making consistent, this case is
> > +	 * checked for and the simple solution is to, in these cases,
> > +	 * decrement prescale and check that the resulting value of period_st=
eps
> > +	 * is valid.
> > +	 *
> > +	 * period_steps can be computed from prescale:
> > +	 *                      period * clk_rate
> > +	 * period_steps =3D ----------------------------- - 1
> > +	 *                NSEC_PER_SEC * (prescale + 1)
> > +	 *
> > +	 */
> > +	if (tmp % (MCHPCOREPWM_PERIOD_STEPS_MAX + 1) < *prescale) {
>=20
> Hmm, looks like 32-bit doesn't like this modulus.
> I pushed things out for LKP to test before sending as I felt I'd not be
> allowed to do that operation, but got a build success email from it.
> I'm not sure why the mail wasn't sent as a reply to this, but
> <202304020410.A86IBNES-lkp@intel.com> complains:
> pwm-microchip-core.c:(.text+0x20a): undefined reference to `__aeabi_uldiv=
mod'
>=20
> I know that tmp < 65536 at this point, so if the general approach is
> fine, I can always cast it to a non 64-bit type without losing any
> information.

Since you already use some of the helpers from linux/math64.h, perhaps
you can use something like div_u64_rem() here?

Thierry

>=20
> > +		u16 smaller_prescale =3D *prescale - 1;
> > +
> > +		*period_steps =3D div_u64(tmp, smaller_prescale + 1) - 1;
> > +		if (*period_steps < 255) {
> > +			*prescale =3D smaller_prescale;
> > +
> > +			return 0;
> > +		}
> > +	}


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