linux-kernel - Re: [PATCH v4 1/4] drm: sun4i: dsi: Use drm_of_find_panel_or

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Message-ID: <YFsIkGH2cRgWk8z9@pendragon.ideasonboard.com>
Date:   Wed, 24 Mar 2021 11:38:24 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     Samuel Holland <samuel@...lland.org>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-amarula <linux-amarula@...rulasolutions.com>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [PATCH v4 1/4] drm: sun4i: dsi: Use drm_of_find_panel_or_bridge

Hi Jagan,

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@...rulasolutions.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@...asonboard.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.

-- 
Regards,

Laurent Pinchart