[alsa-devel] [PATCH] ALSA: ctl: allow TLV read operation for callback type of element in locked case

[Takashi Sakamoto]

On Mon, Dec 23, 2019 at 04:03:53PM +0100, Takashi Iwai wrote:
> On Mon, 23 Dec 2019 10:33:47 +0100,
> Takashi Sakamoto wrote:
> > 
> > A design of ALSA control core allows applications to execute three
> > operations for TLV feature; read, write and command. Furthermore, it
> > allows driver developers to process the operations by two ways; allocated
> > array or callback function. In the former, read operation is just allowed,
> > thus developers uses the latter when device driver supports variety of
> > models or the target model is expected to dynamically change information
> > stored in TLV container.
> > 
> > The core also allows applications to lock any element so that the other
> > applications can't perform write operation to the element for element
> > value and TLV information. When the element is locked, write and command
> > operation for TLV information are prohibited as well as element value.
> > Any read operation should be allowed in the case.
> > 
> > At present, when an element has callback function for TLV information,
> > TLV read operation returns EPERM if the element is locked. On the
> > other hand, the read operation is success when an element has allocated
> > array for TLV information. In both cases, read operation is success for
> > element value expectedly.
> > 
> > This commit fixes the bug. This change can be backported to v4.14
> > kernel or later.
> 
> The patch looks good but your sign-off is missing...

Oops, I was in the festive mood...

Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>

Besides, let us backport this patch to older kernels? As long as I
investigate, this bug exists since v2.6.19 kernel, in which TLV feature
was firstly introduced and extended[1]. When I worked for refactoring to
control core in v4,14 kernel, the bug remains kept as is[2].

It's possible to apply this patch to the longterm v4.14.160 kernel. But
when fixing this bug for the older kernels. we need to prepare an
alternative patch. In my opinion, the disadvantage of the bug is not so
critical, thus it's reasonable to abandon the older kernels.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/commit/?h=8aa9b586e4209
[2] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/commit/?h=450296f305f13


Regards

Takashi Sakamoto