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

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Date:   Fri, 1 Oct 2021 13:37:50 +0530
From:   Pratyush Yadav <p.yadav@...com>
To:     Vinod Koul <vkoul@...nel.org>
CC:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Nikhil Devshatwar <nikhil.nd@...com>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Kishon Vijay Abraham I <kishon@...com>,
        Rob Herring <robh+dt@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-phy@...ts.infradead.org>
Subject: Re: [PATCH v5 2/6] phy: cdns-dphy: Add Rx support

On 01/10/21 11:53AM, Vinod Koul wrote:
> 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..?

Yes, it is due to direction. Different settings need to be applied for 
Rx mode.

> 
> > 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.

Any instance of the underlying IP can only either be TX or RX, it can't 
do both.

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

Ok, will update this patch then.

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.