Re: [PATCH v2] drm/bridge: Add debugfs print for bridge chains

From: Tomi Valkeinen
Date: Mon Jul 31 2023 - 07:42:33 EST

On 25/07/2023 14:37, Laurent Pinchart wrote:
Hi Tomi,

Thank you for the patch.

On Fri, Jul 21, 2023 at 06:01:39PM +0300, Tomi Valkeinen wrote:
DRM bridges are not visible to the userspace and it may not be
immediately clear if the chain is somehow constructed incorrectly. I
have had two separate instances of a bridge driver failing to do a
drm_bridge_attach() call, resulting in the bridge connector not being
part of the chain. In some situations this doesn't seem to cause issues,
but it will if DRM_BRIDGE_ATTACH_NO_CONNECTOR flag is used.

Add a debugfs file to print the bridge chains. For me, on this TI AM62
based platform, I get the following output:

encoder[39]
bridge[0] type: 0, ops: 0x0
bridge[1] type: 0, ops: 0x0, OF: /bus@f0000/i2c@20000000/dsi@e:toshiba,tc358778
bridge[2] type: 0, ops: 0x3, OF: /bus@f0000/i2c@20010000/hdmi@48:lontium,lt8912b
bridge[3] type: 11, ops: 0x7, OF: /hdmi-connector:hdmi-connector

Names would be more readable than numbers, but I'm not sure that's
really worth it. It can always be improved on top if desired.

For type and ops? I agree, but it might also make the output more cluttered. To be honest, I'm not sure if type and ops are useful here, I just felt that I should print something else than just the OF node =).

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx>
---
Changes in v2:
- Fixed compilation issue when !CONFIG_OF
- Link to v1: https://lore.kernel.org/r/20230721-drm-bridge-chain-debugfs-v1-1-8614ff7e890d@xxxxxxxxxxxxxxxx
---
drivers/gpu/drm/drm_bridge.c | 50 +++++++++++++++++++++++++++++++++++++++++++
drivers/gpu/drm/drm_debugfs.c | 3 +++
include/drm/drm_bridge.h | 5 +++++
3 files changed, 58 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index c3d69af02e79..d3eb62d5ef3b 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -27,8 +27,10 @@
#include <linux/mutex.h>
#include <drm/drm_atomic_state_helper.h>
+#include <drm/drm_debugfs.h>
#include <drm/drm_bridge.h>
#include <drm/drm_encoder.h>
+#include <drm/drm_file.h>
#include <drm/drm_of.h>
#include <drm/drm_print.h>
@@ -1345,6 +1347,54 @@ struct drm_bridge *of_drm_find_bridge(struct device_node *np)
EXPORT_SYMBOL(of_drm_find_bridge);
#endif
+#ifdef CONFIG_DEBUG_FS
+static int drm_bridge_chains_info(struct seq_file *m, void *data)
+{
+ struct drm_debugfs_entry *entry = m->private;
+ struct drm_device *dev = entry->dev;
+ struct drm_printer p = drm_seq_file_printer(m);
+ struct drm_mode_config *config = &dev->mode_config;

As Alexander reported, there's a crash for GPU drivers, as mode_config
isn't initialized in that case. I would skip creating the debugfs entry
if DRIVER_MODESET isn't set.

Yes, makes sense.

+ struct drm_encoder *encoder;
+ unsigned int bridge_idx = 0;
+
+ list_for_each_entry(encoder, &config->encoder_list, head) {
+ struct drm_bridge *bridge;
+
+ drm_printf(&p, "encoder[%u]\n", encoder->base.id);
+
+ bridge = drm_bridge_chain_get_first_bridge(encoder);
+
+ while (bridge) {

Would drm_for_each_bridge_in_chain() help ?

Yes.

+ drm_printf(&p, "\tbridge[%u] type: %u, ops: %#x",
+ bridge_idx, bridge->type, bridge->ops);
+
+#ifdef CONFIG_OF
+ if (bridge->of_node)
+ drm_printf(&p, ", OF: %pOFfc", bridge->of_node);
+#endif
+
+ drm_printf(&p, "\n");
+
+ bridge_idx++;
+ bridge = drm_bridge_get_next_bridge(bridge);
+ }
+ }
+
+ return 0;
+}
+
+/* any use in debugfs files to dump individual planes/crtc/etc? */

Those can easily be listed from userspace, so I don't think that's
needed.

Oops. That comment is not supposed to be there. It was a copy-paste error.

+static const struct drm_debugfs_info drm_bridge_debugfs_list[] = {
+ {"bridge_chains", drm_bridge_chains_info, 0},

Missing spaces after '{' and before '}'.

Yep.

+};
+
+void drm_bridge_debugfs_init(struct drm_minor *minor)
+{
+ drm_debugfs_add_files(minor->dev, drm_bridge_debugfs_list,
+ ARRAY_SIZE(drm_bridge_debugfs_list));
+}
+#endif
+
MODULE_AUTHOR("Ajay Kumar <ajaykumar.rs@xxxxxxxxxxx>");
MODULE_DESCRIPTION("DRM bridge infrastructure");
MODULE_LICENSE("GPL and additional rights");
diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
index c90dbcffa0dc..3e89559d68cd 100644
--- a/drivers/gpu/drm/drm_debugfs.c
+++ b/drivers/gpu/drm/drm_debugfs.c
@@ -31,6 +31,7 @@
#include <drm/drm_atomic.h>
#include <drm/drm_auth.h>
+#include <drm/drm_bridge.h>
#include <drm/drm_client.h>
#include <drm/drm_debugfs.h>
#include <drm/drm_device.h>
@@ -272,6 +273,8 @@ int drm_debugfs_init(struct drm_minor *minor, int minor_id,
drm_debugfs_add_files(minor->dev, drm_debugfs_list, DRM_DEBUGFS_ENTRIES);
+ drm_bridge_debugfs_init(minor);
+
if (drm_drv_uses_atomic_modeset(dev)) {
drm_atomic_debugfs_init(minor);
}
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index bf964cdfb330..60dbee6bd1e6 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -949,4 +949,9 @@ static inline struct drm_bridge *drmm_of_get_bridge(struct drm_device *drm,
}
#endif
+#ifdef CONFIG_DEBUG_FS

You could drop the conditional compilation, it wouldn't hurt.

I used the same style as in drm_crtc_internal.h. But you're right, the ifdef doesn't really do much here.

+struct drm_minor;
+void drm_bridge_debugfs_init(struct drm_minor *minor);
+#endif
+
#endif

---
base-commit: c7a472297169156252a50d76965eb36b081186e2
change-id: 20230721-drm-bridge-chain-debugfs-0bbc1522f57a


Tomi