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Date:   Wed, 24 Mar 2021 11:11:09 +0100
From:   Maxime Ripard <maxime@cerno.tech>
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     Jagan Teki <jagan@amarulasolutions.com>,
        Samuel Holland <samuel@sholland.org>,
        Chen-Yu Tsai <wens@csie.org>,
        Jernej Skrabec <jernej.skrabec@siol.net>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-amarula <linux-amarula@amarulasolutions.com>,
        linux-sunxi <linux-sunxi@googlegroups.com>
Subject: Re: [PATCH v4 1/4] drm: sun4i: dsi: Use drm_of_find_panel_or_bridge
Message-ID: <20210324101109.k46k3l7bksvxhll5@gilmour>
References: <20210322140152.101709-1-jagan@amarulasolutions.com>
 <20210322140152.101709-2-jagan@amarulasolutions.com>
 <YFpxYpA+EIZm7sOf@pendragon.ideasonboard.com>
 <f47bc0ad-dbd6-05b5-aaec-2e3256e3715a@sholland.org>
 <CAMty3ZDOVeMeYTsuF8n4EQTG6eEbj6e33TuTPrFiMWG4RhRdSw@mail.gmail.com>
 <YFsIkGH2cRgWk8z9@pendragon.ideasonboard.com>
 <CAMty3ZBGnz_a4_HO_TZ-zPNJwHMcVJyrBi3kZX2=a6G47Ze-yw@mail.gmail.com>
 <YFsMl2CB0ZmpJiKf@pendragon.ideasonboard.com>
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On Wed, Mar 24, 2021 at 11:55:35AM +0200, Laurent Pinchart wrote:
> Hi Jagan,
>=20
> On Wed, Mar 24, 2021 at 03:19:10PM +0530, Jagan Teki wrote:
> > On Wed, Mar 24, 2021 at 3:09 PM Laurent Pinchart wrote:
> > > On Wed, Mar 24, 2021 at 02:44:57PM +0530, Jagan Teki wrote:
> > > > On Wed, Mar 24, 2021 at 8:18 AM Samuel Holland wrote:
> > > > > On 3/23/21 5:53 PM, Laurent Pinchart wrote:
> > > > > > On Mon, Mar 22, 2021 at 07:31:49PM +0530, Jagan Teki wrote:
> > > > > >> Replace of_drm_find_panel with drm_of_find_panel_or_bridge
> > > > > >> for finding panel, this indeed help to find the bridge if
> > > > > >> bridge support added.
> > > > > >>
> > > > > >> Added NULL in bridge argument, same will replace with bridge
> > > > > >> parameter once bridge supported.
> > > > > >>
> > > > > >> Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
> > > > > >
> > > > > > Looks good, there should be no functional change.
> > > > >
> > > > > Actually this breaks all existing users of this driver, see below.
> > > > >
> > > > > > Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.co=
m>
> > > > > >
> > > > > >> ---
> > > > > >> Changes for v4, v3:
> > > > > >> - none
> > > > > >>
> > > > > >>  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 11 ++++++++---
> > > > > >>  1 file changed, 8 insertions(+), 3 deletions(-)
> > > > > >>
> > > > > >> diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/=
gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > > > >> index 4f5efcace68e..2e9e7b2d4145 100644
> > > > > >> --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > > > >> +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > > > >> @@ -21,6 +21,7 @@
> > > > > >>
> > > > > >>  #include <drm/drm_atomic_helper.h>
> > > > > >>  #include <drm/drm_mipi_dsi.h>
> > > > > >> +#include <drm/drm_of.h>
> > > > > >>  #include <drm/drm_panel.h>
> > > > > >>  #include <drm/drm_print.h>
> > > > > >>  #include <drm/drm_probe_helper.h>
> > > > > >> @@ -963,10 +964,14 @@ static int sun6i_dsi_attach(struct mipi_=
dsi_host *host,
> > > > > >>                          struct mipi_dsi_device *device)
> > > > > >>  {
> > > > > >>      struct sun6i_dsi *dsi =3D host_to_sun6i_dsi(host);
> > > > > >> -    struct drm_panel *panel =3D of_drm_find_panel(device->dev=
=2Eof_node);
> > > > >
> > > > > This is using the OF node of the DSI device, which is a direct ch=
ild of
> > > > > the DSI host's OF node. There is no OF graph involved.
> > > > >
> > > > > >> +    struct drm_panel *panel;
> > > > > >> +    int ret;
> > > > > >> +
> > > > > >> +    ret =3D drm_of_find_panel_or_bridge(dsi->dev->of_node, 0,=
 0,
> > > > > >> +                                      &panel, NULL);
> > > > >
> > > > > However, this function expects to find the panel using OF graph. =
This
> > > > > does not work with existing device trees (PinePhone, PineTab) whi=
ch do
> > > > > not use OF graph to connect the panel. And it cannot work, becaus=
e the
> > > > > DSI host's binding specifies a single port: the input port from t=
he
> > > > > display engine.
> > > >
> > > > Thanks for noticing this. I did understand your point and yes, I did
> > > > mention the updated pipeline in previous versions and forgot to add=
 it
> > > > to this series.
> > > >
> > > > Here is the updated pipeline to make it work:
> > > >
> > > > https://patchwork.kernel.org/project/dri-devel/patch/20190524104252=
=2E20236-1-jagan@amarulasolutions.com/
> > > >
> > > > Let me know your comments on this, so I will add a patch for the
> > > > above-affected DTS files.
> > >
> > > DT is an ABI, we need to ensure backward compatibility. Changes in
> > > kernel drivers can't break devices that have an old DT.
> >=20
> > Thanks for your point.
> >=20
> > So, we need to choose APIs that would compatible with the old DT and
> > new DT changes. Am I correct?
>=20
> Yes, that's correct.

However, I see no particular reason to change the DT binding in this
case. The DSI devices are supposed to be described through a subnode of
their DSI controller, that's the generic binding and except for very odd
devices (and a bridge like this one is certainly not one), I see no
reason to deviate from that.

Maxime

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