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]. 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. Supporting hotplug in the latest public draft is based on an ugly workaround in the hotplug-bridge driver code. This is visible in the hotplug_bridge_dsi_attach implementation and documentation in [3] (but keeping in mind that workaround is complicated as it is also circumventing another problem: updating the DSI host format when the DSI device gets connected). As a preliminary step to supporting hotplug in a proper way, and also make this driver cleaner, move drm_bridge_add() at probe time, so that the bridge is available during boot. However simply moving drm_bridge_add() prevents populating the whole card when the hot-pluggable addon is not present at boot, for another reason. 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 This needs to be fixed in the bridge attach function by simply returning -EPROBE_DEFER ifs the following bridge (i.e. the DSI device) is not yet present. [0] https://lpc.events/event/18/contributions/1750/ [1] https://lore.kernel.org/lkml/20240924174254.711c7138@booty/ [2] https://lore.kernel.org/lkml/20250723-drm-bridge-alloc-getput-for_each_bridge-v1-0-be8f4ae00...@bootlin.com/ [3] https://lore.kernel.org/lkml/20240917-hotplug-drm-bridge-v4-4-bc4dfee61...@bootlin.com/ Signed-off-by: Luca Ceresoli <luca.ceres...@bootlin.com> --- drivers/gpu/drm/bridge/samsung-dsim.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c index c4997795db18280903570646b0a5b2c03b666307..173f730edb3707823b0a85460968a11b8206b508 100644 --- a/drivers/gpu/drm/bridge/samsung-dsim.c +++ b/drivers/gpu/drm/bridge/samsung-dsim.c @@ -1633,6 +1633,9 @@ static int samsung_dsim_attach(struct drm_bridge *bridge, { struct samsung_dsim *dsi = bridge_to_dsi(bridge); + if (!dsi->out_bridge) + return -EPROBE_DEFER; + return drm_bridge_attach(encoder, dsi->out_bridge, bridge, flags); } @@ -1749,8 +1752,6 @@ 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. * @@ -2011,6 +2012,8 @@ int samsung_dsim_probe(struct platform_device *pdev) goto err_disable_runtime; } + drm_bridge_add(&dsi->bridge); + return 0; err_disable_runtime: --- base-commit: e48123c607a0db8b9ad02f83c8c3d39918dbda06 change-id: 20250725-drm-bridge-samsung-dsim-add-in-probe-7c549360af90 Best regards, -- Luca Ceresoli <luca.ceres...@bootlin.com>
