Linking ALSA sequencer kernel clients to their device

Commit Message

Martin Koegler Feb. 11, 2016, 7:50 a.m. UTC

In my domain, people use multiple identical USB-MIDI keyboards - each
is assigned a different functions. They expect to kept this mapping
across reboots.

The sound card number are dynamic, therefore they can change after
reboots. The only stable device ID is the sysfs path of the used USB
port.

This means, that I need to associate a kernel sequencer client number
with a sysfs device.

rawmidi devices expose the card number via IOCTLs, which allows to
find the corresponding device in sysfs.

The sequencer provides no identifing data. Chromium works around this
issue by scanning rawmidi as well as sequencer devices and matching
them by using assumtions, how the kernel register sequencer devices.

So I thought about exposing the missing information via sysfs:

From 776dd02d8d7eae89c2699be27eedf81eaf690a0e Mon Sep 17 00:00:00 2001
From: Martin Koegler <[e-mail]>
Date: Mon, 8 Feb 2016 22:53:42 +0100
Subject: [PATCH] Provide sequencer client number via sysfs

Provides the missing link between real hardware devices and kernel
sequencer clients.

Signed-off-by: Martin Koegler <[e-mail]>
---
 sound/core/seq/seq_midi.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

Comments

Clemens Ladisch Feb. 11, 2016, 9:17 a.m. UTC | #1

Martin Koegler wrote:
> In my domain, people use multiple identical USB-MIDI keyboards - each
> is assigned a different functions. They expect to kept this mapping
> across reboots.
>
> The sound card number are dynamic, therefore they can change after
> reboots. The only stable device ID is the sysfs path of the used USB
> port.
>
> This means, that I need to associate a kernel sequencer client number
> with a sysfs device.
>
> rawmidi devices expose the card number via IOCTLs, which allows to
> find the corresponding device in sysfs.
>
> The sequencer provides no identifing data. Chromium works around this
> issue by scanning rawmidi as well as sequencer devices and matching
> them by using assumtions, how the kernel register sequencer devices.

This information wouldn't be wrong in the commit message.

> So I thought about exposing the missing information via sysfs:

The ioctl API (and the library API on top of it) already exists.  Why
do you want to introduce another kind of API that cannot be used for
virtual devices?

Just add the card number to snd_seq_client_info.  (For kernel drivers;
user clients might have the PID here while we're at it.)


Regards,
Clemens

Patch

diff --git a/sound/core/seq/seq_midi.c b/sound/core/seq/seq_midi.c
index 5dd0ee2..0d807e2 100644
--- a/sound/core/seq/seq_midi.c
+++ b/sound/core/seq/seq_midi.c
@@ -271,6 +271,31 @@  static void snd_seq_midisynth_delete(struct seq_midisynth *msynth)
 	snd_midi_event_free(msynth->parser);
 }
 
+static ssize_t
+client_number_show_attr(struct device *dev,
+		     struct device_attribute *attr, char *buf)
+{
+	struct snd_seq_device *device = container_of(dev, struct snd_seq_device, dev);
+	struct snd_card *card = device->card;
+	struct seq_midisynth_client *client;
+
+	client = card ? synths[card->number] : NULL;
+	return snprintf(buf, PAGE_SIZE, "%i\n", client ? client->seq_client : -1);
+}
+
+static DEVICE_ATTR(client, S_IRUGO, client_number_show_attr, NULL);
+
+static ssize_t
+device_number_show_attr(struct device *dev,
+		     struct device_attribute *attr, char *buf)
+{
+	struct snd_seq_device *device = container_of(dev, struct snd_seq_device, dev);
+
+	return snprintf(buf, PAGE_SIZE, "%i\n", device->device);
+}
+
+static DEVICE_ATTR(device, S_IRUGO, device_number_show_attr, NULL);
+
 /* register new midi synth port */
 static int
 snd_seq_midisynth_probe(struct device *_dev)
@@ -287,6 +312,7 @@  snd_seq_midisynth_probe(struct device *_dev)
 	struct snd_card *card = dev->card;
 	int device = dev->device;
 	unsigned int input_count = 0, output_count = 0;
+	int error;
 
 	if (snd_BUG_ON(!card || device < 0 || device >= SNDRV_RAWMIDI_DEVICES))
 		return -EINVAL;
@@ -332,6 +358,16 @@  snd_seq_midisynth_probe(struct device *_dev)
 			kfree(info);
 			return -ENOMEM;
 		}
+		error = device_create_file(&dev->dev, &dev_attr_client);
+		if (error >= 0)
+			error = device_create_file(&dev->dev, &dev_attr_device);
+		if (error < 0) {
+			snd_seq_delete_kernel_client(client->seq_client);
+			kfree(client);
+			mutex_unlock(&register_mutex);
+			kfree(info);
+			return error;
+		}
 	}
 
 	msynth = kcalloc(ports, sizeof(struct seq_midisynth), GFP_KERNEL);
@@ -451,6 +487,8 @@  snd_seq_midisynth_remove(struct device *_dev)
 	kfree(msynth);
 	client->num_ports--;
 	if (client->num_ports <= 0) {
+		device_remove_file(&dev->dev, &dev_attr_client);
+		device_remove_file(&dev->dev, &dev_attr_device);
 		snd_seq_delete_kernel_client(client->seq_client);
 		synths[card->number] = NULL;
 		kfree(client);