linux-kernel - Re: [PATCH v3 5/6] i2c: nomadik: fix BRCR computation

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Message-ID: <CACRpkdagORGgjjhJOxkU445EQ1AGz-4j15E=tGCrvEx7hAnHAQ@mail.gmail.com>
Date: Wed, 9 Oct 2024 17:52:40 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Théo Lebrun <theo.lebrun@...tlin.com>
Cc: Andi Shyti <andi.shyti@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	linux-arm-kernel@...ts.infradead.org, linux-i2c@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Vladimir Kondratiev <vladimir.kondratiev@...ileye.com>, 
	Grégory Clement <gregory.clement@...tlin.com>, 
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>, Tawfik Bayouk <tawfik.bayouk@...ileye.com>
Subject: Re: [PATCH v3 5/6] i2c: nomadik: fix BRCR computation

On Wed, Oct 9, 2024 at 4:01 PM Théo Lebrun <theo.lebrun@...tlin.com> wrote:

> Current BRCR computation is:
>
>     brcr = floor(i2cclk / (clkfreq * div))
>
> With brcr: "baud rate counter", an internal clock divider,
>  and i2cclk: input clock rate (24MHz, 38.4MHz or 48MHz),
>  and clkfreq: desired bus rate,
>  and div: speed-mode dependent divider (2 for standard, 3 otherwise).
>
> Assume i2cclk=48MHz, clkfreq=3.4MHz, div=3,
>   then brcr = floor(48MHz / (3.4MHz * 3)) = 4
>    and resulting bus rate = 48MHz / (4 * 3) = 4MHz
>
> Assume i2cclk=38.4MHz, clkfreq=1.0MHz, div=3,
>   then brcr = floor(38.4MHz / (1.0MHz * 3)) = 12
>    and resulting bus rate = 38.4MHz / (12 * 3) = 1066kHz
>
> The current computation means we always pick the smallest divider that
> gives a bus rate above target. We should instead pick the largest
> divider that gives a bus rate below target, using:
>
>     brcr = floor(i2cclk / (clkfreq * div)) + 1
>
> If we redo the above examples:
>
> Assume i2cclk=48MHz, clkfreq=3.4MHz, div=3,
>   then brcr = floor(48MHz / (3.4MHz * 3)) + 1 = 5
>    and resulting bus rate = 48MHz / (5 * 3) = 3.2MHz
>
> Assume i2cclk=38.4MHz, clkfreq=1.0MHz, div=3,
>   then brcr = floor(38.4MHz / (1.0MHz * 3)) + 1 = 13
>    and resulting bus rate = 38.4MHz / (13 * 3) = 985kHz
>
> In kernel C code, floor(x)   is DIV_ROUND_DOWN() and,
>                   floor(x)+1 is DIV_ROUND_UP().
>
> This is much less of an issue with slower bus rates (ie those currently
> supported), because the gap from one divider to the next is much
> smaller. It however keeps us from always using bus rates superior to
> the target.
>
> This fix is required for later on supporting faster bus rates:
> I2C_FREQ_MODE_FAST_PLUS (1MHz) and I2C_FREQ_MODE_HIGH_SPEED (3.4MHz).
>
> Signed-off-by: Théo Lebrun <theo.lebrun@...tlin.com>

Excellent, thanks for iterating!
Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

Yours,
Linus Walleij