Re: [PATCH] serial: sh-sci: Support custom speed setting

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Eugeniu Rosca <erosca@de.adit-jv.com>,
	"open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	Wolfram Sang <wsa+renesas@sang-engineering.com>,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>,
	Ulrich Hecht <uli+renesas@fpond.eu>,
	"George G . Davis" <george_davis@mentor.com>,
	Andrew Gabbasov <andrew_gabbasov@mentor.com>,
	Jiada Wang <jiada_wang@mentor.com>,
	Yuichi Kusakabe <yuichi.kusakabe@denso-ten.com>,
	Yasushi Asano <yasano@jp.adit-jv.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Jiri Slaby <jslaby@suse.com>,
	Fukui Yohhei <yohhei.fukui@denso-ten.com>,
	Torii Kenichi <torii.ken1@jp.fujitsu.com>,
	Magnus Damm <magnus.damm@gmail.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	linux-man@vger.kernel.org
Subject: Re: [PATCH] serial: sh-sci: Support custom speed setting
Date: Tue, 11 Feb 2020 09:15:02 +0100	[thread overview]
Message-ID: <CAMuHMdUa0fUHZF03QCLsgvS8LSN_rGUQ1gPtotQ3uNGEHkCm6g@mail.gmail.com> (raw)
In-Reply-To: <20200210205735.GB1347752@kroah.com>

Hi Greg,

CC man

On Mon, Feb 10, 2020 at 9:57 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Thu, Jan 30, 2020 at 01:32:50PM +0100, Geert Uytterhoeven wrote:
> > On Wed, Jan 29, 2020 at 5:20 PM Eugeniu Rosca <erosca@de.adit-jv.com> wrote:
> > > From: Torii Kenichi <torii.ken1@jp.fujitsu.com>
> > >
> > > This patch is necessary to use BT module and XM module with DENSO TEN
> > > development board.
> > >
> > > This patch supports ASYNC_SPD_CUST flag by ioctl(TIOCSSERIAL), enables
> > > custom speed setting with setserial(1).
> > >
> > > The custom speed is calculated from uartclk and custom_divisor.
> > > If custom_divisor is zero, custom speed setting is invalid.
> > >
> > > Signed-off-by: Torii Kenichi <torii.ken1@jp.fujitsu.com>
> > > [erosca: rebase against v5.5]
> > > Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
> >
> > Thanks for your patch!
> >
> > While this seems to work fine[*], I have a few comments/questions:
> >   1. This feature seems to be deprecated:
> >
> >          sh-sci e6e68000.serial: setserial sets custom speed on
> > ttySC1. This is deprecated.
> >
> >   2. As the wanted speed is specified as a divider, the resulting speed
> >      may be off, cfr. the example for 57600 below.
> >      Note that the SCIF device has multiple clock inputs, and can do
> >      57600 perfectly if the right crystal has been fitted.
> >
> >  3. What to do with "[PATCH/RFC] serial: sh-sci: Update uartclk based
> >      on selected clock" (https://patchwork.kernel.org/patch/11103703/)?
> >      Combined with this, things become pretty complicated and
> >      unpredictable, as uartclk now always reflect the frequency of the
> >      last used base clock, which was the optimal one for the previously
> >      used speed....
> >
> > I think it would be easier if we just had an API to specify a raw speed.
> > Perhaps that already exists?
>
> Yes, see:
>         http://www.panix.com/~grante/arbitrary-baud.c

Thanks a lot!!
This must be one of the most guarded secrets of serial port programming ;-)

Implemented since 2006, commit edc6afc5496875a6 ("[PATCH] tty: switch to
ktermios and new framework"), not documented in today's man-pages.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds