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

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Message-ID: <20200211123009.GB1858119@kroah.com>
Date:   Tue, 11 Feb 2020 04:30:09 -0800
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Eugeniu Rosca <erosca@...adit-jv.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Ulrich Hecht <uli+renesas@...nd.eu>,
        "George G . Davis" <george_davis@...tor.com>,
        Andrew Gabbasov <andrew_gabbasov@...tor.com>,
        Jiada Wang <jiada_wang@...tor.com>,
        Yuichi Kusakabe <yuichi.kusakabe@...so-ten.com>,
        Yasushi Asano <yasano@...adit-jv.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jiri Slaby <jslaby@...e.com>,
        Fukui Yohhei <yohhei.fukui@...so-ten.com>,
        Torii Kenichi <torii.ken1@...fujitsu.com>,
        Magnus Damm <magnus.damm@...il.com>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        linux-man@...r.kernel.org
Subject: Re: [PATCH] serial: sh-sci: Support custom speed setting

On Tue, Feb 11, 2020 at 09:15:02AM +0100, Geert Uytterhoeven wrote:
> Hi Greg,
> 
> CC man
> 
> On Mon, Feb 10, 2020 at 9:57 PM Greg Kroah-Hartman
> <gregkh@...uxfoundation.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@...adit-jv.com> wrote:
> > > > From: Torii Kenichi <torii.ken1@...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@...fujitsu.com>
> > > > [erosca: rebase against v5.5]
> > > > Signed-off-by: Eugeniu Rosca <erosca@...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 ;-)

It really is, I have to look it up each time it comes up :(

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

yeah, serial port control really needs to be documented better, there's
all sorts of nice ways of controlling them that people don't seem to
know about.  I used to have a link to a bunch of online examples, but
can't seem to find that anymore either.  Ugh, another thing for the long
TODO file...

greg k-h