linux-kernel - Re: [PATCH 04/14] media: sun4i-csi: Fix [HV]sync polarity handling

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Date:   Mon, 16 Jan 2023 11:16:51 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Oleg Verych <olecom@...il.com>
Cc:     wens@...nel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, mark.rutland@....com,
        mchehab@...nel.org, robh+dt@...nel.org,
        sakari.ailus@...ux.intel.com, wens@...e.org
Subject: Re: [PATCH 04/14] media: sun4i-csi: Fix [HV]sync polarity handling

Hi,

On Mon, Jan 16, 2023 at 01:03:59PM +0300, Oleg Verych wrote:
> > -	hsync_pol = !!(bus->flags & V4L2_MBUS_HSYNC_ACTIVE_HIGH);
> > -	vsync_pol = !!(bus->flags & V4L2_MBUS_VSYNC_ACTIVE_HIGH);
> > +	/*
> > +	 * This hardware uses [HV]REF instead of [HV]SYNC. Based on the
> > +	 * provided timing diagrams in the manual, positive polarity
> > +	 * equals active high [HV]REF.
> > +	 *
> > +	 * When the back porch is 0, [HV]REF is more or less equivalent
> > +	 * to [HV]SYNC inverted.
> > +	 */
> > +	href_pol = !!(bus->flags & V4L2_MBUS_HSYNC_ACTIVE_LOW);
> > +	vref_pol = !!(bus->flags & V4L2_MBUS_VSYNC_ACTIVE_LOW);
> 
> After this change has been made there is a need of explicit explanation
> of what "Active high" / "Active low" in dts really mean.

Why?

Active high means that the signal is considered active when it is held
high. Active low means that the signal is considered active when it is
low.

> Currently physical high/low voltage levels are like that:
> (I'm not sure about vsync-active)
> 
> * hsync-active = <0>; /* HSYNC active 'low' => wire active is 'high' */

Yes

>   CSI register setting: href_pol: 1,

Not really, no. It's what this patch commit log is saying: HREF is
!HSYNC, so in order to get a hsync pulse active high, you need to set
href_pol to 0.

> That is confusing:
> 
> [PATCH v6 5/5] DO NOT MERGE: ARM: dts: bananapi: Add Camera support
> https://lore.kernel.org/linux-arm-kernel/cf0e40b0bca9219d2bb023a5b7f23bad8baba1e5.1562847292.git-series.maxime.ripard@bootlin.com/#r
> 
> > +	port {
> > +		csi_from_ov5640: endpoint {
> > +                        remote-endpoint = <&ov5640_to_csi>;
> > +                        bus-width = <8>;
> > +                        hsync-active = <1>; /* Active high */
> 
> original CSI driver
> 
> > +                        vsync-active = <0>; /* Active low */
> > +                        data-active = <1>;  /* Active high */
> > +                        pclk-sample = <1>;  /* Rising */
> > +                };
> > +	};
> 
> [PATCH 13/14] [DO NOT MERGE] ARM: dts: sun7i: cubieboard2: Enable OV7670 camera on CSI1
> https://lore.kernel.org/linux-arm-kernel/20191215165924.28314-14-wens@kernel.org/
> 
> > +	port {
> > +		/* Parallel bus endpoint */
> > +		csi_from_ov7670: endpoint {
> > +			remote-endpoint = <&ov7670_to_csi>;
> > +			bus-width = <8>;
> > +			/* driver is broken */
> > +			hsync-active = <0>; /* Active high */
> 
> this change patchset
> 
> > +			vsync-active = <1>; /* Active high */
> > +			data-active = <1>;  /* Active high */
> > +			pclk-sample = <1>;  /* Rising */
> > +		};
> 
> > +			ov7670_to_csi: endpoint {
> > +				remote-endpoint = <&csi_from_ov7670>;
> > +				bus-width = <8>;
> > +				hsync-active = <1>; /* Active high */
> 
> this patcheset
> 
> > +				vsync-active = <1>; /* Active high */
> > +				data-active = <1>;  /* Active high */
> > +				pclk-sample = <1>;  /* Rising */
> > +			};
> > +		};

I'm sorry, it's not clear to me what is confusing in those excerpts?

Maxime

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