linux-kernel - Re: [PATCH 3/3] USB: cdc-acm: fix TIOCGSERIAL implementation

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Date:   Thu, 8 Apr 2021 13:54:57 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] USB: cdc-acm: fix TIOCGSERIAL implementation

On Thu, Apr 08, 2021 at 01:34:12PM +0200, Oliver Neukum wrote:
> Am Donnerstag, den 08.04.2021, 11:48 +0200 schrieb Johan Hovold:
> > On Thu, Apr 08, 2021 at 10:36:46AM +0200, Oliver Neukum wrote:
> > > Am Mittwoch, den 07.04.2021, 12:28 +0200 schrieb Johan Hovold:
> 
> > > Well, the devices report it. It is part of the standard.
> > 
> > No, the standard doesn't include anything about a baud-base clock
> > AFAICT.
> 
> Unfortunately it does.
> dwDTERate - chapter 6.3.11 - table 17

That's not the base clock rate, that's just the currently configured
line speed which you can read from termios.

> If we does this wrongly, we should certainly fix it, but just removing
> the reporting doesn't look right to me.

The driver got its interpretation of baud_base wrong, and CDC doesn't
even have a concept of base clock rate so removing it is the right thing
to do.

Again, baud_base is really only relevant with legacy UARTs and when
using the deprecated ASYNC_SPD_CUST.

And if the user wants to knows the current line speed we have a
different interface for that.

Johan