linux-kernel - Re: [PATCH 7/9] riscv: dts: spacemit: Add PWM14 backlight support for BPI-F3

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Message-ID: <20250411144549-GYA22568@gentoo>
Date: Fri, 11 Apr 2025 14:45:49 +0000
From: Yixun Lan <dlan@...too.org>
To: Alex Elder <elder@...cstar.com>
Cc: Guodong Xu <guodong@...cstar.com>, ukleinek@...nel.org, robh@...nel.org,
	krzk+dt@...nel.org, conor+dt@...nel.org, paul.walmsley@...ive.com,
	palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr,
	p.zabel@...gutronix.de, drew@...7.com, inochiama@...il.com,
	geert+renesas@...der.be, heylenay@....org, tglx@...utronix.de,
	hal.feng@...rfivetech.com, unicorn_wang@...look.com,
	duje.mihanovic@...le.hr, linux-pwm@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org, spacemit@...ts.linux.dev
Subject: Re: [PATCH 7/9] riscv: dts: spacemit: Add PWM14 backlight support
 for BPI-F3

Hi Alex,

On 09:23 Fri 11 Apr     , Alex Elder wrote:
> On 4/11/25 9:05 AM, Yixun Lan wrote:
> > 
> > On 21:14 Fri 11 Apr     , Guodong Xu wrote:
> >> Add a PWM-based backlight node for the Banana Pi BPI-F3 board,
> >> using PWM14. The backlight is defined as a 'pwm-backlight' device with
> >> brightness levels and a default brightness setting. PWM14 is assigned
> >> a period length of 2000 nanoseconds.
> >>
> >> This configuration was used to verify PWM driver changes, with PWM14
> >> tested and its waveform confirmed as correct.
> >>
> >> The node status is set to "disabled", and should be enabled when the
> >> display driver is ready.
> >>
> > .. see comments below
> >> Signed-off-by: Guodong Xu <guodong@...cstar.com>
> >> ---
> >>   .../boot/dts/spacemit/k1-bananapi-f3.dts      | 32 +++++++++++++++++++
> >>   1 file changed, 32 insertions(+)
> >>
> >> diff --git a/arch/riscv/boot/dts/spacemit/k1-bananapi-f3.dts b/arch/riscv/boot/dts/spacemit/k1-bananapi-f3.dts
> >> index 816ef1bc358e..d04b57ddeb46 100644
> >> --- a/arch/riscv/boot/dts/spacemit/k1-bananapi-f3.dts
> >> +++ b/arch/riscv/boot/dts/spacemit/k1-bananapi-f3.dts
> >> @@ -28,6 +28,32 @@ led1 {
> >>   			default-state = "on";
> >>   		};
> >>   	};
> >> +
> >> +	pwm_bl: lcd_backlight {
> >> +		compatible = "pwm-backlight";
> >> +
> >> +		pwms = <&pwm14 2000>;
> >> +		brightness-levels = <
> >> +			0   40  40  40  40  40  40  40  40  40  40  40  40  40  40  40
> >> +			40  40  40  40  40  40  40  40  40  40  40  40  40  40  40  40
> >> +			40  40  40  40  40  40  40  40  40  41  42  43  44  45  46  47
> >> +			48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63
> >> +			64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79
> >> +			80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95
> >> +			96  97  98  99  100 101 102 103 104 105 106 107 108 109 110 111
> >> +			112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127
> >> +			128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143
> >> +			144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159
> >> +			160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175
> >> +			176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191
> >> +			192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207
> >> +			208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223
> >> +			224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239
> >> +			240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255
> >> +		>;
> >> +		default-brightness-level = <100>;
> >> +		status = "disabled";
> > I'm confused, has DT in board file with disabled status doesn't make sense?
> > it doesn't really useful for placeholder, even worse that functionality may not
> > verified, so I'd suggest sending along with display driver while at it..
> 
> I think I suggested he include this.  Guodong tested PWM using
> a backlight on a display connected to a Banana Pi PBI-F3 board.
> The above numbers come directly from the downstream code, which
> uses this PWM consistently as a display back light.
> 
> But you're right, the exact set of numbers to use is dependent
> on the display used, so it's better to add them when the display
> gets integrated.
> 
Ok
> The pwm14 node could update still be added here, but that too
> might as well wait until there's something to use it.  So I
> think this patch can just be dropped.
> 
thanks for the explanation

-- 
Yixun Lan (dlan)
Gentoo Linux Developer
GPG Key ID AABEFD55