[PATCH v3] mailbox: add list of used channels to debugfs

[PATCH v3] mailbox: add list of used channels to debugfs

[Wolfram Sang]

During development, it is useful to see which mailboxes are currently
obtained. Use a seq-file in debugfs to list the currently registered
controllers and their used channels. Example output from a Renesas R-Car
X5H based system:

189e0000.system-controller:
   0: c1000000.mailbox_test_send_to_recv
   1: c1000100.mailbox_test_recv_to_send
 128: c1000100.mailbox_test_recv_to_send
 129: c1000000.mailbox_test_send_to_recv
189e1000.system-controller:
   4: scmi_dev.1
   5: scmi_dev.2

Note that mailbox controllers currently can be instantiated at any
initcall stage. So, per-controller debugfs handling was discarded
because it is not clear when to create the root "mailbox"-debugfs entry.
A central file was chosen, similar to the GPIO subsystem, which is
independent of the initcall stage because it will be accessed only when
userspace is available anyhow.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
---

Changes since v2:
* rebased on top of linux-next
  (used the wrong branch previously, I am sorry about that!)

 drivers/mailbox/mailbox.c | 57 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 138ffbcd4fde..d3a29e361f33 100644
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -7,6 +7,7 @@
  */
 
 #include <linux/cleanup.h>
+#include <linux/debugfs.h>
 #include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/err.h>
@@ -16,6 +17,7 @@
 #include <linux/mutex.h>
 #include <linux/of.h>
 #include <linux/property.h>
+#include <linux/seq_file.h>
 #include <linux/spinlock.h>
 
 static LIST_HEAD(mbox_cons);
@@ -633,3 +635,58 @@ int devm_mbox_controller_register(struct device *dev,
 	return 0;
 }
 EXPORT_SYMBOL_GPL(devm_mbox_controller_register);
+
+#ifdef CONFIG_DEBUG_FS
+static void *mbox_seq_start(struct seq_file *s, loff_t *pos)
+{
+	mutex_lock(&con_mutex);
+	return seq_list_start(&mbox_cons, *pos);
+}
+
+static void *mbox_seq_next(struct seq_file *s, void *v, loff_t *pos)
+{
+	return seq_list_next(v, &mbox_cons, pos);
+}
+
+static void mbox_seq_stop(struct seq_file *s, void *v)
+{
+	mutex_unlock(&con_mutex);
+}
+
+static int mbox_seq_show(struct seq_file *seq, void *v)
+{
+	const struct mbox_controller *mbox = list_entry(v, struct mbox_controller, node);
+
+	seq_printf(seq, "%s:\n", dev_name(mbox->dev));
+
+	for (unsigned int i = 0; i < mbox->num_chans; i++) {
+		struct mbox_chan *chan = &mbox->chans[i];
+
+		scoped_guard(spinlock_irqsave, &chan->lock) {
+			if (chan->cl) {
+				struct device *cl_dev = chan->cl->dev;
+
+				seq_printf(seq, " %3u: %s\n", i,
+					   cl_dev ? dev_name(cl_dev) : "NULL device");
+			}
+		}
+	}
+
+	return 0;
+}
+
+static const struct seq_operations mbox_sops = {
+	.start = mbox_seq_start,
+	.next = mbox_seq_next,
+	.stop = mbox_seq_stop,
+	.show = mbox_seq_show,
+};
+DEFINE_SEQ_ATTRIBUTE(mbox);
+
+static int __init mbox_init(void)
+{
+	debugfs_create_file("mailbox_summary", 0444, NULL, NULL, &mbox_fops);
+	return 0;
+}
+subsys_initcall(mbox_init);
+#endif	/* DEBUG_FS */
-- 
2.51.0

Re: [PATCH v3] mailbox: add list of used channels to debugfs

[Geert Uytterhoeven]

Hi Wolfram,

On Mon, 13 Apr 2026 at 14:09, Wolfram Sang
<wsa+renesas@sang-engineering.com> wrote:
> During development, it is useful to see which mailboxes are currently
> obtained. Use a seq-file in debugfs to list the currently registered
> controllers and their used channels. Example output from a Renesas R-Car
> X5H based system:
>
> 189e0000.system-controller:
>    0: c1000000.mailbox_test_send_to_recv
>    1: c1000100.mailbox_test_recv_to_send
>  128: c1000100.mailbox_test_recv_to_send
>  129: c1000000.mailbox_test_send_to_recv
> 189e1000.system-controller:
>    4: scmi_dev.1
>    5: scmi_dev.2
>
> Note that mailbox controllers currently can be instantiated at any
> initcall stage. So, per-controller debugfs handling was discarded
> because it is not clear when to create the root "mailbox"-debugfs entry.
> A central file was chosen, similar to the GPIO subsystem, which is
> independent of the initcall stage because it will be accessed only when
> userspace is available anyhow.
>
> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Thanks for your patch, which looks useful!

> --- a/drivers/mailbox/mailbox.c
> +++ b/drivers/mailbox/mailbox.c

> +static int __init mbox_init(void)
> +{
> +       debugfs_create_file("mailbox_summary", 0444, NULL, NULL, &mbox_fops);

Can you please put it in a subdir, like all other summaries:

    ~ # ls  /sys/kernel/debug/*/*summary
    /sys/kernel/debug/clk/clk_orphan_summary
    /sys/kernel/debug/clk/clk_summary
    /sys/kernel/debug/devfreq/devfreq_summary
    /sys/kernel/debug/dmaengine/summary
    /sys/kernel/debug/pm_genpd/pm_genpd_summary
    /sys/kernel/debug/regulator/regulator_summary

? More mailbox-related files might be added later.
(But more importantly, that way my summary grabbing script will save
 it automatically ;-)

> +       return 0;
> +}
> +subsys_initcall(mbox_init);
> +#endif /* DEBUG_FS */

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH v3] mailbox: add list of used channels to debugfs

[Wolfram Sang]

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Hi Geert,

> > Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
> 
> Thanks for your patch, which looks useful!

Glad you like it!

> Can you please put it in a subdir, like all other summaries:

Sadly not, because of the paragraph from above:

> > Note that mailbox controllers currently can be instantiated at any
> > initcall stage. So, per-controller debugfs handling was discarded
> > because it is not clear when to create the root "mailbox"-debugfs entry.
> > A central file was chosen, similar to the GPIO subsystem, which is
> > independent of the initcall stage because it will be accessed only when
> > userspace is available anyhow.

I would need to use core_initcall for the mailbox-subsys. debugfs itself
also uses core_initcall. Then, some drivers (e.g. hi3660-mailbox) use
core_initcall. I don't think the subdir is worth all the potential race
conditions and fragile solutions which usually come with it. Or do you
know a solid solution to this problem?

Happy hacking,

   Wolfram


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Re: [PATCH v3] mailbox: add list of used channels to debugfs

[Geert Uytterhoeven]

Hi Wolfram,

On Tue, 14 Apr 2026 at 17:28, Wolfram Sang
<wsa+renesas@sang-engineering.com> wrote:
> > > Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
> >
> > Thanks for your patch, which looks useful!
>
> Glad you like it!
>
> > Can you please put it in a subdir, like all other summaries:
>
> Sadly not, because of the paragraph from above:
>
> > > Note that mailbox controllers currently can be instantiated at any
> > > initcall stage. So, per-controller debugfs handling was discarded
> > > because it is not clear when to create the root "mailbox"-debugfs entry.
> > > A central file was chosen, similar to the GPIO subsystem, which is
> > > independent of the initcall stage because it will be accessed only when
> > > userspace is available anyhow.
>
> I would need to use core_initcall for the mailbox-subsys. debugfs itself
> also uses core_initcall. Then, some drivers (e.g. hi3660-mailbox) use
> core_initcall. I don't think the subdir is worth all the potential race
> conditions and fragile solutions which usually come with it. Or do you
> know a solid solution to this problem?

I am not asking for a per-controller subdir, but a mailbox-specific
subdir. Surely you can do something like:

-       debugfs_create_file("mailbox_summary", 0444, NULL, NULL, &mbox_fops);
+       mbox_debugfs = debugfs_create_dir("mailbox", NULL);
+       debugfs_create_file("mailbox_summary", 0444, mbox_debugfs,
+                           NULL, &mbox_fops);

Cfr. https://elixir.bootlin.com/linux/v7.0/source/drivers/devfreq/devfreq.c#L2036

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds