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

From: Thomas Zimmermann <tzimmermann@suse.de>
To: Arnd Bergmann <arnd@arndb.de>, kernel test robot <lkp@intel.com>,
	Helge Deller <deller@gmx.de>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Javier Martinez Canillas <javierm@redhat.com>,
	Daniel Vetter <daniel@ffwll.ch>, Vineet Gupta <vgupta@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	"David S . Miller" <davem@davemloft.net>,
	"James E . J . Bottomley" <James.Bottomley@hansenpartnership.com>,
	Sam Ravnborg <sam@ravnborg.org>,
	suijingfeng@loongson.cn
Cc: Linux-Arch <linux-arch@vger.kernel.org>,
	linux-fbdev@vger.kernel.org, linux-ia64@vger.kernel.org,
	linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org, linux-m68k@lists.linux-m68k.org,
	loongarch@lists.linux.dev, oe-kbuild-all@lists.linux.dev,
	sparclinux@vger.kernel.org, linux-snps-arc@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v6 5/6] fbdev: Move framebuffer I/O helpers into <asm/fb.h>
Date: Thu, 11 May 2023 15:12:06 +0200	[thread overview]
Message-ID: <cee24e80-cc41-00bc-06a2-37f1e2cad8ef@suse.de> (raw)
In-Reply-To: <743d2b1e-c843-4fb2-b252-0006be2e2bd8@app.fastmail.com>

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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
> 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.

It just occured to me that these fb_read and fb_write calls are probably 
all wrong. The fb_ interfaces are for framebuffer I/O memory. The driver 
uses them to access the regular state registers. The writew() on sh is 
definitely different. [1]

I assume that it only works because CONFIG_SWAP_IO_SPACE [2] is not set 
in hp6xx_defconfig.

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/latest/source/arch/sh/include/asm/io.h#L55
[2] 
https://elixir.bootlin.com/linux/latest/source/arch/sh/include/mach-common/mach/mangle-port.h#L22

> 
>        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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