linux-kernel - Re: [PATCH v2] pwm: berlin: Don't use broken prescaler values

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Date:   Mon, 9 Jul 2018 18:57:24 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Thomas Hebb <tommyhebb@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Antoine Tenart <antoine.tenart@...e-electrons.com>,
        sebastian.hesselbarth@...il.com, Jisheng.Zhang@...aptics.com,
        "open list:PWM SUBSYSTEM" <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH v2] pwm: berlin: Don't use broken prescaler values

On Wed, Jun 06, 2018 at 01:42:10PM -0400, Thomas Hebb wrote:
> The Berlin PWM driver is currently broken on at least BG2CD. The
> symptoms manifest as a very non-linear and erratic mapping from the duty
> cycle configured in software to the duty cycle produced by hardware.
> 
> The cause of the bug is software's configuration of the prescaler, and
> in particular its usage of the six prescaler values between the minimum
> value of 1 and the maximum value of 4096. As it turns out, these six
> values do not actually slow down the PWM clock; rather, they emulate
> slowing down the clock by internally multiplying the value of TCNT.
> 
> This would be a fine trick, if not for the fact that the internal,
> scaled TCNT value has no extra bits beyond the 16 already exposed to
> software in the register. What this means is that, for a prescaler of 4,
> the software must ensure that the top two bits of TCNT are not set,
> because hardware will chop them off; for a prescaler of 8, the top three
> bits must not be set, and so forth. Software does not currently ensure
> this, resulting in a TCNT several orders of magnitude lower than
> intended any time one of those six prescalers are selected.
> 
> Because hardware chops off the high bits in its internal shift, the
> middle six prescalers don't actually allow *anything* that the first
> doesn't. In fact, they are strictly worse than the first, since the
> internal shift of TCNT prevents software from setting the low bits,
> decreasing the resolution, without providing any extra high bits.
> 
> By skipping the useless prescalers entirely, this patch both fixes the
> driver's behavior and increases its performance (since, when the 4096
> prescaler is selected, it now does only a single shift rather than the
> seven successive divisions it did before).
> 
> Tested on BG2CD.
> 
> Signed-off-by: Thomas Hebb <tommyhebb@...il.com>
> ---
>  drivers/pwm/pwm-berlin.c | 45 ++++++++++++++++++++++------------------
>  1 file changed, 25 insertions(+), 20 deletions(-)

Applied, thanks.

Thierry

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