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

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Date:   Thu, 03 Mar 2022 10:22:59 +0000
From:   Paul Cercueil <paul@...pouillou.net>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Jiri Slaby <jslaby@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        'Uwe Kleine-König' 
        <u.kleine-koenig@...gutronix.de>, gregkh@...uxfoundation.org,
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        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
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        Paul Mackerras <paulus@...ba.org>,
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        Baolin Wang <baolin.wang7@...il.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Andreas Färber <afaerber@...e.de>
Subject: Re: [PATCH v3] serial: make uart_console_write->putchar()'s character
 an unsigned char

Hi Maciej,

Le jeu., mars 3 2022 at 09:55:17 +0000, Maciej W. Rozycki 
<macro@...am.me.uk> a écrit :
> On Thu, 3 Mar 2022, Jiri Slaby wrote:
> 
>>  > 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).
>> 
>>  So should we use int/uint instead of more appropriate shorter types 
>> everywhere
>>  now? The answer is: definitely not. The assembly on x86 looks good 
>> (it uses
>>  movz, no ands), RISC architectures have to do what they chose to.
> 
>  We do have an issue, because we still have this:
> 
> void uart_console_write(struct uart_port *port, const char *s,
> 			unsigned int count,
> 			void (*putchar)(struct uart_port *, int))
> 
> and then:
> 
> 		putchar(port, *s);
> 
> there.  Consequently on targets where plain `char' type is signed the
> value retrieved from `*s' has to be truncated in the call to 
> `putchar'.
> And indeed it happens with the MIPS target:
> 
> 803ae47c:	82050000 	lb	a1,0(s0)
> 803ae480:	26100001 	addiu	s0,s0,1
> 803ae484:	02402025 	move	a0,s2
> 803ae488:	0220f809 	jalr	s1
> 803ae48c:	30a500ff 	andi	a1,a1,0xff
> 
> vs current code:
> 
> 803ae47c:	82050000 	lb	a1,0(s0)
> 803ae480:	26100001 	addiu	s0,s0,1
> 803ae484:	0220f809 	jalr	s1
> 803ae488:	02402025 	move	a0,s2

And how is that at all a problem?

> (NB the last instruction shown after the call instruction, JALR, is 
> in the
> delay slot that is executed before the PC gets updated).  Now 
> arguably the
> compiler might notice that and use an unsigned LBU load instruction 
> rather
> than the signed LB load instruction, which would make the ANDI 
> instruction
> redundant, but still I think we ought to avoid gratuitous type 
> signedness
> changes.
> 
>  So I'd recommend changing `s' here to `const unsigned char *' or, as 
> I
> previously suggested, maybe to `const u8 *' even.

Just cast the string to "const u8 *" within the function, while keeping 
a "const char *s" argument. The compiler will then most likely generate 
LBUs.

Cheers,
-Paul