linux-kernel - Re: [PATCH 2/7] memory: atmel-ebi: Simplify SMC config code

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Message-ID: <16727676.CMj9rWYKWZ@ada>
Date:   Thu, 02 Mar 2017 13:02:16 +0100
From:   Alexander Dahl <ada@...rsis.com>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Nicolas Ferre <nicolas.ferre@...el.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Lee Jones <lee.jones@...aro.org>,
        Samuel Ortiz <sameo@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] memory: atmel-ebi: Simplify SMC config code

Hei hei,

With 

#define ATMEL_SMC_MODE_TDF(x)           (((x) - 1) << 16)

from include/linux/mfd/syscon/atmel-smc.h you added this:

> +	ret = of_property_read_u32(np, "atmel,smc-tdf-ns", &val);
> +	if (!ret) {
> +		required = true;
> +		ncycles = DIV_ROUND_UP(val, clk_period_ns);
> +		if (ncycles > ATMEL_SMC_MODE_TDF_MAX) {
> +			ret = -EINVAL;
> +			goto out;
> +		}

[…]

> +		smcconf->mode |= ATMEL_SMC_MODE_TDF(ncycles);
> +	}

This was the same algorithm at some other location in atmel-ebi.c 
before:

	#define AT91_SMC_TDF_(x)		((((x) - 1) << 16) & AT91_SMC_TDF)

	val = DIV_ROUND_UP(timings->tdf_ns, clk_rate);
	if (val > AT91_SMC_TDF_MAX)
		val = AT91_SMC_TDF_MAX;
	regmap_fields_write(fields->mode, conf->cs,
			    config->mode | AT91_SMC_TDF_(val));

The hardware manual (AT91SAM9G20) says values from 0 to 15 (4bit, 0x0 to 
0xF) are possible and I guess the goal is to set it to a value 
corresponding to the value in ns from the dts or to 15 if it's greater 
(or -EINVAL in the new version).

However how can one set it to zero? Put in zero to the div you get zero 
for ncycles or val and that goes as x into (((x) - 1) << 16) which 
results in 0xF ending up as TDF_CYCLES in the mode register, right?

I can of course set a slightly greater value, which ends up in a 
calculated register value of zero, but that seems more a hack to me and 
is not obvious if I just look at the DTS.

If I'm right this might be topic of another bugfix patch, or should it 
be done right in a v2 of this one?

Greets
Alex