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

From: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
To: Julien Stephan <jstephan@baylibre.com>
Cc: krzysztof.kozlowski@linaro.org, robh@kernel.org,
	chunkuang.hu@kernel.org, linux-mediatek@lists.infradead.org,
	Phi-bang Nguyen <pnguyen@baylibre.com>,
	Louis Kuo <louis.kuo@mediatek.com>,
	Chunfeng Yun <chunfeng.yun@mediatek.com>,
	Vinod Koul <vkoul@kernel.org>,
	Kishon Vijay Abraham I <kishon@kernel.org>,
	Andy Hsieh <andy.hsieh@mediatek.com>,
	Philipp Zabel <p.zabel@pengutronix.de>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	open list <linux-kernel@vger.kernel.org>,
	"moderated list:ARM/Mediatek USB3 PHY DRIVER"
	<linux-arm-kernel@lists.infradead.org>,
	"open list:GENERIC PHY FRAMEWORK" <linux-phy@lists.infradead.org>,
	"open list:DRM DRIVERS FOR MEDIATEK"
	<dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH v2 2/2] phy: mtk-mipi-csi: add driver for CSI phy
Date: Mon, 15 May 2023 16:22:38 +0200	[thread overview]
Message-ID: <85500bcc-c5e8-8ce2-edea-233de86c2d35@collabora.com> (raw)
In-Reply-To: <ynrvqt24hjgng25r2xa3hxj35cvgotx7sdfrbqfjcvj3foegmr@4lqhen5yu6fh>

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

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