linux-kernel - [PATCH 1/3] Revert "USB: cdc-acm: fix rounding error in TIOCSSERIAL"

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Date:   Wed, 7 Apr 2021 13:04:31 +0200
From:   Anthony Mallet <anthony.mallet@...s.fr>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Oliver Neukum <oneukum@...e.com>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: [PATCH 1/3] Revert "USB: cdc-acm: fix rounding error in TIOCSSERIAL"

On Wednesday  7 Apr 2021, at 12:28, Johan Hovold wrote:
> With HZ=250, the default 0.5 second value of close_delay is converted to
> 125 jiffies when set and is converted back to 50 centiseconds by
> TIOCGSERIAL as expected (not 12 cs as was claimed).

It was "12" (instead of 50) because the conversion gor TIOCGSERIAL was
initially broken, and that was fixed in the previous commit
633e2b2ded739a34bd0fb1d8b5b871f7e489ea29

> For completeness: With different default values for these parameters or
> with a HZ value not divisible by two, the lack of rounding when setting
> the default values in tty_port_init() could result in an -EPERM being
> returned, but this is hardly something we need to worry about.

The -EPERM is harmful when a regular user wants to update other
members of serial_struct without changing the close delays,
e.g. ASYNC_LOW_LATENCY, which is granted to regular users.

Best,
Anthony