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

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Message-ID: <YG2WuTPjPhLPR/v7@hovoldconsulting.com>
Date:   Wed, 7 Apr 2021 13:25:45 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Anthony Mallet <anthony.mallet@...s.fr>
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: Re: [PATCH 1/3] Revert "USB: cdc-acm: fix rounding error in
 TIOCSSERIAL"

On Wed, Apr 07, 2021 at 01:04:31PM +0200, Anthony Mallet wrote:
> 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

Right, so this patch is still just broken. The missing jiffies
conversion had already been added.

> > 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.

You're missing the point; -EPERM will *not* be returned -- and this
patch was never needed.

Johan