Re: [PATCH v5 2/6] phy: cdns-dphy: Add Rx support

From: Vinod Koul <vkoul@kernel.org>
To: Pratyush Yadav <p.yadav@ti.com>
Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
	Vignesh Raghavendra <vigneshr@ti.com>,
	Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Nikhil Devshatwar <nikhil.nd@ti.com>,
	Chunfeng Yun <chunfeng.yun@mediatek.com>,
	Kishon Vijay Abraham I <kishon@ti.com>,
	Rob Herring <robh+dt@kernel.org>,
	linux-kernel@vger.kernel.org, linux-phy@lists.infradead.org
Subject: Re: [PATCH v5 2/6] phy: cdns-dphy: Add Rx support
Date: Fri, 1 Oct 2021 11:53:16 +0530	[thread overview]
Message-ID: <YVapVLnGfSBZCDTY@matsya> (raw)
In-Reply-To: <20210917172809.rjtf7ww7vjcfvey5@ti.com>

Hi Pratyush,

On 17-09-21, 22:58, Pratyush Yadav wrote:
> +Rob
> 
> On 16/09/21 12:22PM, Paul Kocialkowski wrote:
> > Hi,
> > 
> > On Fri 03 Sep 21, 00:25, Pratyush Yadav wrote:
> > > The Cadence DPHY can be used to receive image data over the CSI-2
> > > protocol. Add support for Rx mode. The programming sequence differs from
> > > the Tx mode so it is added as a separate set of hooks to isolate the two
> > > paths. The mode in which the DPHY has to be used is selected based on
> > > the compatible.
> > 
> > I just realized that I didn't follow-up on a previous revision on the debate
> > about using the phy sub-mode to distinguish between rx/tx.
> > 
> > I see that you've been using a dedicated compatible, but I'm not sure this is a
> > good fit either. My understanding is that the compatible should describe a group
> > of register-compatible revisions of a hardware component, not how the hardware
> > is used specifically. I guess the distinction between rx/tx falls under
> > the latter rather than the former.
> 
> I am not sure if that is the case. For example, we use "ti,am654-ospi" 
> for Cadence Quadspi controller. The default compatible, "cdns,qspi-nor", 
> only supports Quad SPI (4 lines). The "ti,am654-ospi" compatible also 
> supports Octal SPI (8 lines).

Those are hardware defaults right?

> In addition, I feel like the Rx DPHY is almost a different type of 
> device from a Tx DPHY. The programming sequence is completely different, 

Is that due to direction or something else..?

> the clocks required are different, etc. So I think using a different 
> compatible for Rx mode makes sense.

Is the underlaying IP not capable of both TX and RX and in the specific
situations you are using it as TX and RX.

I am okay that default being TX but you can use Paul's approach of
direction with this to make it better proposal

-- 
~Vinod