linux-kernel - Re: [RFC, PATCH] imx: serial: Take tty->files

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Date:   Tue, 30 May 2017 14:42:13 +0100
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Rob Herring <robherring2@...il.com>
Cc:     Andrey Smirnov <andrew.smirnov@...il.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Chris Healy <cphealy@...il.com>,
        Peter Senna Tschudin <peter.senna@...labora.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC, PATCH] imx: serial: Take tty->files_lock
 opportunistically

> > Fixes: 18a4208826dd0a13eb06de724c86bba2c225f943 ("imx-serial: Reduce
> > RX DMA startup latency when opening for reading")
> >
> > Cc: cphealy@...il.com
> > Cc: Peter Senna Tschudin <peter.senna@...labora.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Jiri Slaby <jslaby@...e.com>
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Andrey Smirnov <andrew.smirnov@...il.com>
> > ---
> >
> > Not sure if this is the best way to solve the problem (hence the RFC
> > tag). If anyone has a better idea, or if there's a better fix for this
> > already, please let me know.  
> 
> IMO, the low level serial drivers shouldn't be accessing
> tty->tty_files in the first place. Is being opened for write-only that
> common and is skipping the DMA setup really necessary?

Seconded - the Reduce RX DMA startup latency patch should just be
reverted (and shouldn't ever IMHO have gotten in).

Not all readers and writers to a tty have a file handle any more anyway,
so it's not only icky and layer violating it's fundamentally broken
beyond the locking.

Alan