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Date:   Fri, 8 Oct 2021 13:27:53 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Pratyush Yadav <p.yadav@ti.com>
Cc:     Vinod Koul <vkoul@kernel.org>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Tomi Valkeinen <tomi.valkeinen@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
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Hi Pratyush,

On Thu, Oct 07, 2021 at 05:44:38PM +0530, Pratyush Yadav wrote:
> On 07/10/21 03:10AM, Laurent Pinchart wrote:
> > On Fri, Oct 01, 2021 at 11:53:16AM +0530, Vinod Koul wrote:
> > > On 17-09-21, 22:58, Pratyush Yadav wrote:
> > > > On 16/09/21 12:22PM, Paul Kocialkowski wrote:
> > > > > 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
> > 
> > 
> > Given that the RX and TX implementations are very different (it's not a
> > matter of selecting a mode at runtime), I'm actually tempted to
> > recommend having two drivers, one for the RX PHY and one for the TX PHY.
> > This can only be done with two different compatible strings, which I
> > think would be a better approach.
> 
> FWIW, I think having different drivers would certainly make things 
> easier to maintain.

I'm sorry for not having recommended this in the first place.

Any objection from anyone against going in this direction ?

> > It's unfortunate that the original compatible string didn't contain
> > "tx". We could rename it and keep the old one in the driver for backward
> > compatibility, making things cleaner going forward.

-- 
Regards,

Laurent Pinchart