linux-kernel - Re: [PATCH v6 5/6] fbdev: Move framebuffer I/O helpers into <asm/fb.h>

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Message-ID: <9ea087ab-95f3-3274-b464-ef13718562b1@suse.de>
Date:   Thu, 11 May 2023 12:53:43 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Arnd Bergmann <arnd@...db.de>, kernel test robot <lkp@...el.com>,
        Helge Deller <deller@....de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Vineet Gupta <vgupta@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        "David S . Miller" <davem@...emloft.net>,
        "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
        Sam Ravnborg <sam@...nborg.org>, suijingfeng@...ngson.cn
Cc:     Linux-Arch <linux-arch@...r.kernel.org>,
        linux-fbdev@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-m68k@...ts.linux-m68k.org,
        loongarch@...ts.linux.dev, oe-kbuild-all@...ts.linux.dev,
        sparclinux@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 5/6] fbdev: Move framebuffer I/O helpers into
 <asm/fb.h>

Hi

Am 10.05.23 um 17:54 schrieb Arnd Bergmann:
> On Wed, May 10, 2023, at 16:27, Thomas Zimmermann wrote:
>> Am 10.05.23 um 16:15 schrieb Arnd Bergmann:
>>> On Wed, May 10, 2023, at 16:03, kernel test robot wrote:
>   
>>> I think that's a preexisting bug and I have no idea what the
>>> correct solution is. Looking for HD64461 shows it being used
>>> both with inw/outw and readw/writew, so there is no way to have
>>> the correct type. The sh __raw_readw() definition hides this bug,
>>> but that is a problem with arch/sh and it probably hides others
>>> as well.
>>
>> The constant HD64461_IOBASE is defined as integer at
>>
>>
>> https://elixir.bootlin.com/linux/latest/source/arch/sh/include/asm/hd64461.h#L17
>>
>> but fb_readw() expects a volatile-void pointer. I guess we could add a
>> cast somewhere to silence the problem. In the current upstream code,
>> that appears to be done by sh's __raw_readw() internally:
>>
>>
>> https://elixir.bootlin.com/linux/latest/source/arch/sh/include/asm/io.h#L35
> 
> Sure, that would make it build again, but that still doesn't make the
> code correct, since it's completely unclear what base address the

Oh, OK. I thought it's like vgafb, which grabs the fixed VGA framebuffer 
range.

> HD64461_IOBASE is relative to. The hp6xx platform code only passes it
> through inw()/outw(), which take an offset relative to sh_io_port_base,
> but that is not initialized on hp6xx. I tried to find in the history
> when it broke, apparently that was in 2007 commit 34a780a0afeb ("sh:
> hp6xx pata_platform support."), which removed the custom inw/outw
> implementations.

Thanks for looking this up. If this driver has been broken for 15 years, 
the correct fix is to delete it. I've meanwhile prepared the second-best 
fix, which is the type casting. It'll be in the next patchset.

Best regards
Thomas

> 
>        Arnd

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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