linux-kernel - Re: [PATCH v3 1/7] phy: Distinguish between Rx and Tx for MIPI D-PHY with submodes

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Message-ID: <20210811173629.gse5qmzezlh2ua7g@ti.com>
Date:   Wed, 11 Aug 2021 23:06:31 +0530
From:   Pratyush Yadav <p.yadav@...com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
CC:     Vinod Koul <vkoul@...nel.org>,
        Nikhil Devshatwar <nikhil.nd@...com>,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Kishon Vijay Abraham I <kishon@...com>,
        <linux-kernel@...r.kernel.org>, <linux-phy@...ts.infradead.org>
Subject: Re: [PATCH v3 1/7] phy: Distinguish between Rx and Tx for MIPI D-PHY
 with submodes

On 06/08/21 01:11PM, Paul Kocialkowski wrote:
> Hi,
> 
> On Fri 25 Jun 21, 00:11, Pratyush Yadav wrote:
> > From: Paul Kocialkowski <paul.kocialkowski@...tlin.com>
> > 
> > As some D-PHY controllers support both Rx and Tx mode, we need a way for
> > users to explicitly request one or the other. For instance, Rx mode can
> > be used along with MIPI CSI-2 while Tx mode can be used with MIPI DSI.
> > 
> > Introduce new MIPI D-PHY PHY submodes to use with PHY_MODE_MIPI_DPHY.
> > The default (zero value) is kept to Tx so only the rkisp1 driver, which
> > uses D-PHY in Rx mode, needs to be adapted.
> 
> After some thinking and discussions, it appears that using the submode is
> probably not the best way to distinguish between rx and tx. This is because
> rx/tx is more of a specification of the hardware component than a run-time
> descision. Indeed the D-PHY blocks are usually dedicated to an associated
> controller (DSI or CSI-2) and thus each instance is either meant for tx or
> rx use.

Ok. IIRC Laurent also brought this point up earlier as well. At that 
time I took the allwinner DPHY as an example of a DPHY that can run in 
both modes.

The Cadence DPHY also supports both Rx and Tx modes but I don't know if 
both can be implemented at the same time, if that would even make any 
sense.

> 
> As a result I will be using the allwinner,direction string property in
> device-tree instead of the submode, with values of either "rx" or "tx".
> I suppose you can do something similar if you agree it makes more sense.

Wouldn't a different compatible a better idea?

> 
> Cheers,
> 
> Paul
>  
> > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@...tlin.com>
> > Signed-off-by: Pratyush Yadav <p.yadav@...com>
> > ---
> > 
> > (no changes since v1)
> > 
> >  include/linux/phy/phy-mipi-dphy.h | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/include/linux/phy/phy-mipi-dphy.h b/include/linux/phy/phy-mipi-dphy.h
> > index a877ffee845d..0f57ef46a8b5 100644
> > --- a/include/linux/phy/phy-mipi-dphy.h
> > +++ b/include/linux/phy/phy-mipi-dphy.h
> > @@ -6,6 +6,19 @@
> >  #ifndef __PHY_MIPI_DPHY_H_
> >  #define __PHY_MIPI_DPHY_H_
> >  
> > +/**
> > + * enum phy_mipi_dphy_submode - MIPI D-PHY sub-mode
> > + *
> > + * A MIPI D-PHY can be used to transmit or receive data.
> > + * Since some controllers can support both, the direction to enable is specified
> > + * with the PHY sub-mode. Transmit is assumed by default with phy_set_mode.
> > + */
> > +
> > +enum phy_mipi_dphy_submode {
> > +	PHY_MIPI_DPHY_SUBMODE_TX = 0,
> > +	PHY_MIPI_DPHY_SUBMODE_RX,
> > +};
> > +
> >  /**
> >   * struct phy_configure_opts_mipi_dphy - MIPI D-PHY configuration set
> >   *
> > -- 
> > 2.30.0
> > 
> 
> -- 
> Paul Kocialkowski, Bootlin
> Embedded Linux and kernel engineering
> https://bootlin.com



-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.