linux-kernel - Re: [PATCH v7 10/13] serial: sh-sci: Add support for RZ/T2H SCI

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Message-ID: <CAMuHMdV=1Hoo4b8jmKV2sE0VG2E4g=EUus_gfE8hdMvLag1pew@mail.gmail.com>
Date: Mon, 14 Apr 2025 16:07:23 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Thierry Bultel <thierry.bultel@...atsea.fr>, 
	Thierry Bultel <thierry.bultel.yh@...renesas.com>, 
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>, 
	Paul Barker <paul.barker.ct@...renesas.com>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
Subject: Re: [PATCH v7 10/13] serial: sh-sci: Add support for RZ/T2H SCI

Hi Greg,

On Mon, 14 Apr 2025 at 11:32, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Mon, Apr 14, 2025 at 11:23:34AM +0200, Thierry Bultel wrote:
> > Le 14/04/2025 à 10:58, Greg KH a écrit :
> > > On Mon, Apr 14, 2025 at 07:54:12AM +0000, Thierry Bultel wrote:
> > > > > From: Greg KH <gregkh@...uxfoundation.org>
 > > > > On Thu, Apr 03, 2025 at 11:29:12PM +0200, Thierry Bultel wrote:
> > > > > > --- a/include/uapi/linux/serial_core.h
> > > > > > +++ b/include/uapi/linux/serial_core.h
> > > > > > @@ -231,6 +231,9 @@
> > > > > >   /* Sunplus UART */
> > > > > >   #define PORT_SUNPLUS        123
> > > > > >
> > > > > > +/* SH-SCI */
> > > > > > +#define PORT_RSCI    124
> > > > > Why do you need to tell userspace about this specific port?  Is that a
> > > > > hard requirement that your userspace tools require?  If not, please don't
> > > > > export this here.
> > > > This point has been discussed with Geert and Wolfram.
> > > > We cannot use PORT_GENERIC for this IP, and adding the new type
> > > > is just keeping consistent with the sh-sci driver.
> > > But, why does userspace need to know this number?  And why doesn't
> > > PORT_GENERIC work?
> >
> > The reason is that the sh-sci driver discriminates internally between port
> > types.
> > There are number of locations when it checks for PORT_SCI, PORT_SCIF,
> > PORT_SCIFA...
>
> That is internal to the kernel, not external, right?

Indeed.

> > T2H SCI needs special handling, too, that is the reason why PORT_GENERIC
> > cannot work. I just therefore added this new type.
>
> Again, why does userspace need to know this?

I guess userspace does not need to know about this (does userspace
actually care about any of these values, except for PORT_8250 and
friends?).

However, traditionally PORT_* has been used as the differentiator
for serial port types in most drivers in the kernel (incl. sh-sci.c)
as well.  Just two drivers use PORT_GENERIC, and these are the ones
that came with the introduction of PORT_GENERIC.  Since the rsci driver
is not a new driver, but a sub-driver of the existing sh-sci driver,
I think it warrants keeping on using the type field, like before. Else
it will have to become even more complex than it already is.

Note I am fine with using a non-uapi value. Is there a PORT_* number
range meant for internal (non-uapi) use?
Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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