linux-kernel - Re: [PATCH 1/2] serial: sh-sci: Fix early deassertion of dedicated RTS

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Date:   Mon, 09 Jan 2017 13:57:40 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Christoph Baumann <cbaumann@...teon.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] serial: sh-sci: Fix early deassertion of dedicated RTS

Hi Geert,

On Monday 09 Jan 2017 10:53:55 Geert Uytterhoeven wrote:
> On Fri, Jan 6, 2017 at 1:30 PM, Laurent Pinchart wrote:
> > On Friday 02 Dec 2016 13:35:10 Geert Uytterhoeven wrote:
> >> If a UART has dedicated RTS/CTS pins, there are some issues:
> >> 1. When changing hardware control flow, the new AUTORTS state is not
> >>    immediately reflected in the hardware, but only when RTS is raised.
> >>    However, the serial core doesn't call .set_mctrl() after
> >>    .set_termios(), hence AUTORTS may only become effective when the port
> >>    is closed, and reopened later.
> >>    Note that this problem does not happen when manually using stty to
> >>    change CRTSCTS, as AUTORTS will work fine on next open.
> >> 
> >> 2. When hardware control flow is disabled (or AUTORTS is not yet
> >>    effective), changing any serial port configuration deasserts RTS, as
> >>    .set_termios() calls sci_init_pins().
> > 
> > Isn't this still a problem with this patch applied ? Calling
> > sci_set_mctrl() should reconfigure the pins properly, but won't there be
> > a short window during which the configuration will be wrong ?
> 
> You mean in between the calls to sci_init_pins() and sci_set_mctrl()?

That's what I mean, yes.

> I don't think _de_asserting RTS for a few microseconds matters much, would
> it? Asserting RTS wrongly would be worse.

It's not the end of the world, but as you're trying to fix this issue, maybe 
it would be a good time to refactor the code properly and fix the core problem 
(.set_termios() calling sci_init_pins()) rather than working around it :-)

-- 
Regards,

Laurent Pinchart