[PATCH AUTOSEL 6.16 41/85] pwm: sifive: Fix PWM algorithm and clarify inverted compare behavior

[Uwe Kleine-König]

Hello,

On Sun, Aug 03, 2025 at 08:22:50PM -0400, Sasha Levin wrote:
> From: Nylon Chen <nylon.chen at sifive.com>
> 
> [ Upstream commit 7dbc4432ea6bf9d709391eb57f1e9fb44e99845a ]
> 
> The `frac` variable represents the pulse inactive time, and the result
> of this algorithm is the pulse active time. Therefore, we must reverse
> the result.
> 
> Although the SiFive Reference Manual states "pwms >= pwmcmpX -> HIGH",
> the hardware behavior is inverted due to a fixed XNOR with 0. As a result,
> the pwmcmp register actually defines the low (inactive) portion of the pulse.
> 
> The reference is SiFive FU740-C000 Manual[0]
> 
> Link: https://sifive.cdn.prismic.io/sifive/1a82e600-1f93-4f41-b2d8-86ed8b16acba_fu740-c000-manual-v1p6.pdf [0]
> 
> Co-developed-by: Zong Li <zong.li at sifive.com>
> Signed-off-by: Zong Li <zong.li at sifive.com>
> Co-developed-by: Vincent Chen <vincent.chen at sifive.com>
> Signed-off-by: Vincent Chen <vincent.chen at sifive.com>
> Signed-off-by: Nylon Chen <nylon.chen at sifive.com>
> Link: https://lore.kernel.org/r/20250529035341.51736-3-nylon.chen@sifive.com
> Signed-off-by: Uwe Kleine-König <ukleinek at kernel.org>
> Signed-off-by: Sasha Levin <sashal at kernel.org>

Please drop this patch, see
https://lore.kernel.org/all/52ycm5nf2jrxdmdmcijz57xhm2twspjmmiign6zq6rp3d5wt6t@tq5w47fmiwgg/
for the rationale.

Best regards
Uwe
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