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Date:   Fri, 8 Oct 2021 14:55:54 +0200
From:   Paul Kocialkowski <paul.kocialkowski@bootlin.com>
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     Pratyush Yadav <p.yadav@ti.com>, Vinod Koul <vkoul@kernel.org>,
        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,

On Fri 08 Oct 21, 13:27, Laurent Pinchart wrote:
> Hi Pratyush,
>=20
> 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 C=
SI-2
> > > > > > > protocol. Add support for Rx mode. The programming sequence d=
iffers from
> > > > > > > the Tx mode so it is added as a separate set of hooks to isol=
ate the two
> > > > > > > paths. The mode in which the DPHY has to be used is selected =
based on
> > > > > > > the compatible.
> > > > > >=20
> > > > > > 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.
> > > > > >=20
> > > > > > I see that you've been using a dedicated compatible, but I'm no=
t 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 h=
ow the hardware
> > > > > > is used specifically. I guess the distinction between rx/tx fal=
ls under
> > > > > > the latter rather than the former.
> > > > >=20
> > > > > I am not sure if that is the case. For example, we use "ti,am654-=
ospi"=20
> > > > > for Cadence Quadspi controller. The default compatible, "cdns,qsp=
i-nor",=20
> > > > > only supports Quad SPI (4 lines). The "ti,am654-ospi" compatible =
also=20
> > > > > supports Octal SPI (8 lines).
> > > >=20
> > > > Those are hardware defaults right?
> > > >=20
> > > > > In addition, I feel like the Rx DPHY is almost a different type o=
f=20
> > > > > device from a Tx DPHY. The programming sequence is completely dif=
ferent,=20
> > > >=20
> > > > Is that due to direction or something else..?
> > > >=20
> > > > > the clocks required are different, etc. So I think using a differ=
ent=20
> > > > > compatible for Rx mode makes sense.
> > > >=20
> > > > Is the underlaying IP not capable of both TX and RX and in the spec=
ific
> > > > situations you are using it as TX and RX.
> > > >=20
> > > > I am okay that default being TX but you can use Paul's approach of
> > > > direction with this to make it better proposal
> > >=20
> > >=20
> > > 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 P=
HY.
> > > This can only be done with two different compatible strings, which I
> > > think would be a better approach.
> >=20
> > FWIW, I think having different drivers would certainly make things=20
> > easier to maintain.
>=20
> I'm sorry for not having recommended this in the first place.
>=20
> Any objection from anyone against going in this direction ?

So apparently there is not a single register that is shared between rx and =
tx
and clocks are not the same either so it feels to me like a separate driver
would be legit. This looks like two distinct IPs sharing the same base addr=
ess.

Cheers,

Paul

> > > 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 backw=
ard
> > > compatibility, making things cleaner going forward.
>=20
> --=20
> Regards,
>=20
> Laurent Pinchart

--=20
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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