Re: users of drivers/misc/echo ?

["Dr. David Alan Gilbert" <linux@treblig.org>]

* Harald Welte (laforge@gnumonks.org) wrote:
> Hi Dave, Arnd, Greg,
> 
> On Tue, Feb 25, 2025 at 01:01:06PM +0000, Dr. David Alan Gilbert wrote:
> > > However, those DAHDI-using deployments that I personally am familiar
> > > with do not use the software echo canceller discussed here.  On the
> > > other hand, I'm quite certain that there are many PBX/IVR related
> > > systems out there (unrelated to my area of cellular or trunked radio)
> > > that would still use the echo canceller discussed here.
> 
> I have to correct myself here:  "that would still use software echo cancellation".
> 
> As I stated before, in "my" deployments, the echo canceller is not used
> and hence I'm not super familiar with it.  My use cases either don't
> need echo cancellation, or use hardware echo cancellation.
> 
> Revisiting the DAHDI source code as a result of this thread: There are
> actually several different software echo cancellation implementations,
> only one of which (oslec) is using the drivers/misc/echo.
> 
> > Some questions:
> > 
> > 1) I see drivers/dahdi/dahdi_echocan_oslec.c
> > 
> > /* Fix this if OSLEC is elsewhere */
> > #include "../staging/echo/oslec.h"
> > //#include <linux/oslec.h>
> > 
> > now that moved to drivers/misc in 2014 by Greg's
> > 6e2055a9e56e ("staging: echo: move to drivers/misc/")
> > 
> > So is most of this on ancient kernels or do people
> > actually use modern stuff?
> 
> Actually, looking at DAHDI, I really don't think anyone is still using
> the dahdi_echocan_oslec code.  It is disabled by default and only built
> if explicitly enabled by the user, and indeed if anyone did that it
> would fail to build for any kernels that have moved it out of staging.

It looks like Debian is including and enabling it in it's DKMS build:

# apt install dahdi-dkms
...
dahdi_echocan_oslec.ko:
Running module version sanity check.
 - Original module
   - No original module exists within this kernel
 - Installation
   - Installing to /lib/modules/6.1.0-31-amd64/updates/dkms/
...
# nm /lib/modules/6.1.0-31-amd64/updates/dkms/dahdi_echocan_oslec.ko
...
                 U oslec_create
                 U oslec_free
                 U oslec_update
...

> > 2) I see there is a fir.h that's different from the kernels
> > drivers/misc/echo/fir.h  doesn't that cause problems?
> > Should one get updated from the other somehow?
> 
> I'll not investigate that given the above determination.
> 
> > 3) Any idea why it's never been upstreamed?
> 
> I can only speculate, but I guess it was never a strong priority for the authors,
> and indeed likely the code would have had to undergo quite some changes.
> 
> DAHDI is clearly well beyond its peak user base these days, and I would
> expect the amount of effort into mainlining it, together with the
> associated risk of introducing bugs during required refactoring simply
> doesn't make it an attractive proposition.  Also, given that userspace
> applications for it have been around for decades, it would be impossible
> to address any upstreaming related change requests without rendering
> those applications incompatible.
> 
> So I'd really not bother at this point anymore.  The few adjustments
> I/we had to make to keep it building + working with recent kernels over
> the past few years are minimal, and mostly trivial stuff like minor
> kernel API changes.  In the end, DAHDI doesn't interact with a lot of
> other kernel.  It talks to the hardware via its own device drivers, and
> it talks to userspace programs via character devices.  So unless some
> core kernel memory allocator, or PCI or USB device drive APIs or
> character device API changes, we're mostly good.

OK.

Dave

> -- 
> - Harald Welte <laforge@gnumonks.org>          https://laforge.gnumonks.org/
> ============================================================================
> "Privacy in residential applications is a desirable marketing option."
>                                                   (ETSI EN 300 175-7 Ch. A6)
> 
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