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

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Message-ID: <9c4be198444e9987c826c87b592e9dc6@artur-rojek.eu>
Date:   Thu, 11 May 2023 15:22:13 +0200
From:   Artur Rojek <contact@...ur-rojek.eu>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        kernel test robot <lkp@...el.com>,
        Helge Deller <deller@....de>,
        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,
        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 2023-05-11 14:35, Geert Uytterhoeven wrote:
> Hi Arnd,
> 
> CC Artur, who's working on HP Jornada 680.
Thanks for CC'ing me - I faced this exact issue while working on my
(still not upstreamed) hd6446x PCMCIA controller driver. The PCMCIA
subsystem uses `inb/outb`, which expect the `sh_io_port_base` to be set
to something else than the default `-1`. At first I tried to set it to
`0xa0000000`, so that all I/O goes through the fixed, non-cacheable P2
area. That however broke some other driver code (I had no time to debug
which one). Eventually I ended up taking a suggestion from a MIPS PCMCIA
driver [1] and simply substract the broken `sh_io_port_base` address
from `HD64461_IOBASE`, as the base for `socket.io_offset`. This way all
the PCMCIA `inb/outb` accesses are absolute, no matter what the
`sh_io_port_base` is set to. This of course is a very ugly solution and
we should instead fix the root cause of this mess. I will have a better
look at this patch set and the problem at hand at a later date.

Cheers,
Artur

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pcmcia/db1xxx_ss.c?h=v6.4-rc1#n527

> 
> On Wed, May 10, 2023 at 5:55 PM Arnd Bergmann <arnd@...db.de> wrote:
>> 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.
> 
> See also commit 4aafae27d0ce73f8 ("sh: hd64461 tidying."), which
> claims they are no longer needed.
> 
> Don't the I/O port macros just treat the port as an absolute base 
> address
> when sh_io_port_base isn't set?
> 
> Gr{oetje,eeting}s,
> 
>                         Geert