linux-kernel - Re: [PATCH v2 2/2] phy: mtk-mipi-csi: add driver for CSI phy

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Message-ID: <jx2ndeoli3ozjiydygwclrpqmwftgf2gv4kfr6dyclf2cyh4hk@fnhvqeef5w4f>
Date:   Mon, 15 May 2023 16:52:05 +0200
From:   Julien Stephan <jstephan@...libre.com>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
Cc:     krzysztof.kozlowski@...aro.org, robh@...nel.org,
        chunkuang.hu@...nel.org, linux-mediatek@...ts.infradead.org,
        Phi-bang Nguyen <pnguyen@...libre.com>,
        Louis Kuo <louis.kuo@...iatek.com>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Andy Hsieh <andy.hsieh@...iatek.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/Mediatek USB3 PHY DRIVER" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:GENERIC PHY FRAMEWORK" <linux-phy@...ts.infradead.org>,
        "open list:DRM DRIVERS FOR MEDIATEK" 
        <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v2 2/2] phy: mtk-mipi-csi: add driver for CSI phy

On Mon, May 15, 2023 at 04:22:38PM +0200, AngeloGioacchino Del Regno wrote:
> Il 15/05/23 15:36, Julien Stephan ha scritto:
> > On Mon, May 15, 2023 at 02:22:52PM +0200, AngeloGioacchino Del Regno wrote:
> > > Il 15/05/23 11:05, Julien Stephan ha scritto:
> >   ..snip..
> > > > +	port->is_cdphy = of_property_read_bool(dev->of_node, "mediatek,is_cdphy");
> > >
> > > This driver doesn't support C-PHY mode, so you either add support for that, or in
> > > my opinion you should simply refuse to probe it, as it is *dysfunctional* for the
> > > unsupported case (and might even introduce unstabilities).
> > >
> > > 	/* At the moment, only D-PHY mode is supported */
> > > 	if (!port->is_cdphy)
> > > 		return -EINVAL;
> > >
> > > Also, please don't use underscores for devicetree properties: "mediatek,is-cdphy"
> > > is fine.
> > >
> > Hi Angelo,
> > You are right this driver does not support C-PHY mode, but some of the
> > PHYs themselves support BOTH C-PHY AND D-PHY. The idea of `is_cdphy` variable
> > is to know if the CSI port supports BOTH C-PHY AND D-PHY or only DPHY.
> > For example mt8365 has 2 PHYs: CSI0 and CSI1. CSI1 support only D-PHY,
> > while CSI0 can be configured in C-PHY or D-PHY. Registers for CD-PHY and
> > D-PHY are almost identical, except that CD-PHY compatible has some extra
> > bitfields to configure properly the mode and the lanes (because supporting
> > trios for CD-PHY).
> > If C-PHY support is eventually added into the driver, I think we will need
> > another variable such as `mode` to know the mode. I was also thinking
> > of adding a phy argument to determine if the mode is C-PHY or D-PHY.
> >
> > So here, I don't want to stop the probe if `is_cdphy` variable is set to
> > true. Does it make sense ?
> >
>
> Comments in the code convinced me that the other PHYs providing only C or D PHY
> support weren't compatible at all with this driver.
>
> I got it now - but at this point can you please add a comment in the code actually
> clarifying that this driver supports both PHYs providing *only* D-PHY and ones
> providing selectable C-or-D PHY?
>
> That clarified, it would not make sense to stop probing if it's not a CDPHY because
> as you said there might be a D-only PHY that would be actually supported here.
>
> Regards,
> Angelo
>
>
Ok, I will add a comment in the code to make it more clear.

Regards
Julien