[PATCH RFC v2 07/24] drm/bridge: samsung-dsim: move drm_bridge_add() call to probe

[Luca Ceresoli]

This bridge driver calls drm_bridge_add() in the DSI host .attach callback
instead of in the probe function. This looks strange, even though
apparently not a problem for currently supported use cases.

However it is a problem for supporting hotplug of DRM bridges, which is in
the works [0][1][2][3]. The problematic case is when this DSI host is
always present while its DSI device is hot-pluggable. In such case with the
current code the DRM card will not be populated until after the DSI device
attaches to the host, and which could happen a very long time after
booting, or even not happen at all.

The reason is that the previous pipeline component (the encoder in this
case) when probing cannot find the samsung-dsim bridge. What happens is:

 [1 and 2 can happen in any order, same result]
 1) samsung-dsim probes (does not drm_bridge_add() itself)
 2) The lcdif starts probing multiple times, but
    lcdif_probe
    -> lcdif_load
       -> lcdif_attach_bridge
          -> devm_drm_of_get_bridge() returns -EPROBE_DEFER because
             the samsung-dsim is not in the global bridge_list
             (deferred probe pending: imx-lcdif: Cannot connect bridge)

The samsung-dsim will not drm_bridge_add() itself until a DSI device will
try to mipi_dsi_attach() to the DSI Host, which can happen arbitratily late
or never on hot-pluggable hardware.

As a preliminary step to supporting hotplug move drm_bridge_add() at probe
time, so that the samsung-dsim DSI host bridge is available during boot,
even without a connected DSI device. This results in:

 1) samsung-dsim probes (and adds to drm_bridge_add() itself)
 2) The lcdif starts probing multiple times, but
    lcdif_probe
    -> lcdif_load
       -> lcdif_attach_bridge
          -> devm_drm_of_get_bridge() --> OK, returns samsung-dsim ptr
          -> drm_bridge_attach()
             -> samsung_dsim_attach()
                -> drm_bridge_attach()
		   -> -EINVAL because dsi->bridge.next_bridge is still NULL

So moving drm_bridge_add() allows one step further but it is not
enough. The reason is:

 * now the encoder driver finds this bridge instead of getting
   -EPROBE_DEFER as before
 * but it cannot attach it because the bridge attach function in turn tries
   to attach to the following bridge, which has not yet been hot-plugged

Solve this by returning 0 in the bridge attach function in case the
following bridge (i.e. the DSI device) is not yet present. In other words,
for the samsung-dsim bridge it is OK to not have a following bridge. It can
be hotplugged later on.

[0] https://lpc.events/event/18/contributions/1750/
[1] https://www.youtube.com/watch?v=C8dEQ4OzMnc
[2] https://lore.kernel.org/lkml/20240924174254.711c7138@booty/
[3] https://lore.kernel.org/lkml/20260507-drm-bridge-alloc-getput-panel_or_bridge-v5-0-472b913b5cb7@xxxxxxxxxxx/

Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>

---

This patch is similar to [4] but different in code and with a largely
rewritten commit message.

[4] https://lore.kernel.org/lkml/20250725-drm-bridge-samsung-dsim-add-in-probe-v1-1-b23d29c23fbd@xxxxxxxxxxx/
---
 drivers/gpu/drm/bridge/samsung-dsim.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
index dc3ff880d7ac..6c48404fd60a 100644
--- a/drivers/gpu/drm/bridge/samsung-dsim.c
+++ b/drivers/gpu/drm/bridge/samsung-dsim.c
@@ -1827,6 +1827,9 @@ static int samsung_dsim_attach(struct drm_bridge *bridge,
 {
 	struct samsung_dsim *dsi = bridge_to_dsi(bridge);
 
+	if (!dsi->bridge.next_bridge)
+		return 0;
+
 	return drm_bridge_attach(encoder, dsi->bridge.next_bridge, bridge,
 				 flags);
 }
@@ -1965,8 +1968,6 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
 		     mipi_dsi_pixel_format_to_bpp(device->format),
 		     device->mode_flags);
 
-	drm_bridge_add(&dsi->bridge);
-
 	/*
 	 * This is a temporary solution and should be made by more generic way.
 	 *
@@ -1976,7 +1977,7 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
 	if (!(device->mode_flags & MIPI_DSI_MODE_VIDEO)) {
 		ret = samsung_dsim_register_te_irq(dsi, &device->dev);
 		if (ret)
-			goto err_remove_bridge;
+			return ret;
 	}
 
 	// The next bridge can be used by host_ops->attach
@@ -1998,8 +1999,6 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
 	drm_bridge_clear_and_put(&dsi->bridge.next_bridge);
 	if (!(device->mode_flags & MIPI_DSI_MODE_VIDEO))
 		samsung_dsim_unregister_te_irq(dsi);
-err_remove_bridge:
-	drm_bridge_remove(&dsi->bridge);
 	return ret;
 }
 
@@ -2016,8 +2015,6 @@ static int samsung_dsim_host_detach(struct mipi_dsi_host *host,
 
 	samsung_dsim_unregister_te_irq(dsi);
 
-	drm_bridge_remove(&dsi->bridge);
-
 	return 0;
 }
 
@@ -2216,6 +2213,8 @@ int samsung_dsim_probe(struct platform_device *pdev)
 			goto err_disable_runtime;
 	}
 
+	drm_bridge_add(&dsi->bridge);
+
 	return 0;
 
 err_disable_runtime:
@@ -2229,6 +2228,8 @@ void samsung_dsim_remove(struct platform_device *pdev)
 {
 	struct samsung_dsim *dsi = platform_get_drvdata(pdev);
 
+	drm_bridge_remove(&dsi->bridge);
+
 	pm_runtime_disable(&pdev->dev);
 
 	if (dsi->plat_data->host_ops && dsi->plat_data->host_ops->unregister_host)

-- 
2.55.0