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Date:   Sat, 1 Apr 2023 21:50:47 +0100
From:   Conor Dooley <conor@kernel.org>
To:     Conor Dooley <conor.dooley@microchip.com>
Cc:     Thierry Reding <thierry.reding@gmail.com>,
        Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
        <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: <0b91dee7-6c1d-4a33-8235-8fd5d58b200e@spud>
References: <20230330071203.286972-1-conor.dooley@microchip.com>
 <20230330071203.286972-2-conor.dooley@microchip.com>
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On Thu, Mar 30, 2023 at 08:12:03AM +0100, Conor Dooley wrote:

> +	/*
> +	 * 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_steps
> +	 * is valid.
> +	 *
> +	 * period_steps can be computed from prescale:
> +	 *                      period * clk_rate
> +	 * period_steps = ----------------------------- - 1
> +	 *                NSEC_PER_SEC * (prescale + 1)
> +	 *
> +	 */
> +	if (tmp % (MCHPCOREPWM_PERIOD_STEPS_MAX + 1) < *prescale) {

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_uldivmod'

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.

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

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