[PATCH 7/7] ALSA: doc: Add device-managed resource section

From: Takashi Iwai <tiwai@suse.de>
To: alsa-devel@alsa-project.org
Subject: [PATCH 7/7] ALSA: doc: Add device-managed resource section
Date: Thu, 20 Sep 2018 17:55:02 +0200	[thread overview]
Message-ID: <20180920155502.7534-8-tiwai@suse.de> (raw)
In-Reply-To: <20180920155502.7534-1-tiwai@suse.de>

Give brief explanations about the device-managed resources and the
newly introduced snd_devm_card_new() helper.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 .../kernel-api/writing-an-alsa-driver.rst     | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/Documentation/sound/kernel-api/writing-an-alsa-driver.rst b/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
index b60a26807420..b0ca01f3de81 100644
--- a/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
+++ b/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
@@ -4112,6 +4112,39 @@ module license as GPL, etc., otherwise the system is shown as “tainted”.
   MODULE_LICENSE("GPL");
 
 
+Device-Managed Resources
+========================
+
+In the examples above, all resources are allocated and released
+manually.  But human beings are lazy in nature, especially developers
+are lazier.  So there are some ways to automate the release part; it's
+the (device-)managed resources aka devres or devm family.  For
+example, an object allocated via :c:func:`devm_kmalloc()` will be
+freed automatically at unbinding the device.
+
+ALSA core provides also the device-managed helper, namely,
+:c:func:`snd_devm_card_new()` for creating a card object.
+Call this functions instead of the normal :c:func:`snd_card_new()`,
+and you can forget the explicit :c:func:`snd_card_free()` call, as
+it's called automagically at error and removal paths.
+
+One caveat is that the call of :c:func:`snd_card_free()` would be put
+at the beginning of the call chain only after you call
+:c:func:`snd_card_register()`.
+
+Also, the ``private_free`` callback is always called at the card free,
+so be careful to put the hardware clean-up procedure in
+``private_free`` callback.  It might be called even before you
+actually set up at an earlier error path.  For avoiding such an
+invalid initialization, you can set ``private_free`` callback after
+:c:func:`snd_card_register()` call succeeds.
+
+Another thing to be remarked is that you should use device-managed
+helpers for each component as much as possible once when you manage
+the card in that way.  Mixing up with the normal and the managed
+resources may screw up the release order.
+
+
 How To Put Your Driver Into ALSA Tree
 =====================================
 
-- 
2.18.0

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