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Date:   Wed, 24 Mar 2021 11:55:35 +0200
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Jagan Teki <jagan@amarulasolutions.com>
Cc:     Samuel Holland <samuel@sholland.org>,
        Maxime Ripard <mripard@kernel.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: <YFsMl2CB0ZmpJiKf@pendragon.ideasonboard.com>
References: <20210322140152.101709-1-jagan@amarulasolutions.com>
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 <f47bc0ad-dbd6-05b5-aaec-2e3256e3715a@sholland.org>
 <CAMty3ZDOVeMeYTsuF8n4EQTG6eEbj6e33TuTPrFiMWG4RhRdSw@mail.gmail.com>
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Hi Jagan,

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.com>
> > > > >
> > > > >> ---
> > > > >> 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 = host_to_sun6i_dsi(host);
> > > > >> -    struct drm_panel *panel = of_drm_find_panel(device->dev.of_node);
> > > >
> > > > This is using the OF node of the DSI device, which is a direct child of
> > > > the DSI host's OF node. There is no OF graph involved.
> > > >
> > > > >> +    struct drm_panel *panel;
> > > > >> +    int ret;
> > > > >> +
> > > > >> +    ret = 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) which do
> > > > not use OF graph to connect the panel. And it cannot work, because the
> > > > DSI host's binding specifies a single port: the input port from the
> > > > 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/20190524104252.20236-1-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.
> 
> Thanks for your point.
> 
> So, we need to choose APIs that would compatible with the old DT and
> new DT changes. Am I correct?

Yes, that's correct.

-- 
Regards,

Laurent Pinchart