linux-kernel - RE: [PATCH v3] serial: make uart_console_write->putchar()'s character an unsigned char

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Date:   Wed, 2 Mar 2022 23:57:44 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Uwe Kleine-König' 
        <u.kleine-koenig@...gutronix.de>, Jiri Slaby <jslaby@...e.cz>
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Subject: RE: [PATCH v3] serial: make uart_console_write->putchar()'s character
 an unsigned char

From: Uwe Kleine-König
> Sent: 02 March 2022 17:53
> 
> On Wed, Mar 02, 2022 at 08:27:32AM +0100, Jiri Slaby wrote:
> > Currently, uart_console_write->putchar's second parameter (the
> > character) is of type int. It makes little sense, provided uart_console_write()
> > accepts the input string as "const char *s" and passes its content -- the
> > characters -- to putchar(). So switch the character's type to unsigned
> > char.
> >
> > We don't use char as that is signed on some platforms. That would cause
> > troubles for drivers which (implicitly) cast the char to u16 when
> > writing to the device. Sign extension would happen in that case and the
> > value written would be completely different to the provided char. DZ is
> > an example of such a driver -- on MIPS, it uses u16 for dz_out in
> > dz_console_putchar().
> 
> I always thought this was bigger than 8bit for hardware that supports
> wider characters. But if that's the case that's completely unsupported,
> there isn't even CS9.

The real problem is that using char (or short) for a function parameter
or result is very likely to require the compile add code to mask
the value to 8 (or 16) bits.

Remember that almost every time you do anything with a signed or unsigned
char/short variable the compiler has to use the integer promotion rules
to convert the value to int.

You'll almost certainly get better code if the value is left in an
int (or unsigned int) variable until the low 8 bits get written to
a buffer (or hardware register).

	David

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