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

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Message-Id: <743d2b1e-c843-4fb2-b252-0006be2e2bd8@app.fastmail.com>
Date:   Wed, 10 May 2023 17:54:18 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Thomas Zimmermann" <tzimmermann@...e.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:     oe-kbuild-all@...ts.linux.dev,
        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, 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>

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
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.

      Arnd