linux-kernel - Re: [PATCH 1/1] pwm: Convert period and duty cycle to u64

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Message-ID: <20191016101539.GC1303817@ulmo>
Date:   Wed, 16 Oct 2019 12:15:39 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Guru Das Srinagesh <gurus@...eaurora.org>
Cc:     linux-pwm@...r.kernel.org, kernel-team@...roid.com,
        Mark Salyzyn <salyzyn@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] pwm: Convert period and duty cycle to u64

On Tue, Oct 15, 2019 at 07:11:39PM -0700, Guru Das Srinagesh wrote:
> Because period and duty cycle are defined as ints with units of
> nanoseconds, the maximum time duration that can be set is limited to
> ~2.147 seconds. Change their definitions to u64 so that higher durations
> may be set.
> 
> Signed-off-by: Guru Das Srinagesh <gurus@...eaurora.org>
> ---
>  drivers/pwm/core.c  |  4 ++--
>  drivers/pwm/sysfs.c | 10 +++++-----
>  include/linux/pwm.h | 16 ++++++++--------
>  3 files changed, 15 insertions(+), 15 deletions(-)

Actually, we can't do that without further preparatory work. The reason
is that consumers use the period and duty_cycle members in computations
of their own, which lead to errors such as this:

	armv7l-unknown-linux-gnueabihf-ld: drivers/video/backlight/pwm_bl.o: in function `pwm_backlight_probe':
	pwm_bl.c:(.text+0x3b0): undefined reference to `__aeabi_uldivmod'

So I think we need to audit all consumers carefully and make sure that
they use do_div() where necessary to avoid such errors.

Thierry

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