linux-kernel - Re: [PATCH 0/5] fbdev: Move framebuffer I/O helpers to <asm/fb.h>

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Message-Id: <173a9a6c-d47d-487c-b61a-66d021548f5a@app.fastmail.com>
Date:   Thu, 27 Apr 2023 11:20:32 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Thomas Zimmermann" <tzimmermann@...e.de>,
        "Sam Ravnborg" <sam@...nborg.org>
Cc:     "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>,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-ia64@...r.kernel.org, loongarch@...ts.linux.dev,
        linux-m68k@...ts.linux-m68k.org, sparclinux@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-parisc@...r.kernel.org
Subject: Re: [PATCH 0/5] fbdev: Move framebuffer I/O helpers to <asm/fb.h>

On Thu, Apr 27, 2023, at 08:22, Thomas Zimmermann wrote:
> Am 26.04.23 um 21:21 schrieb Sam Ravnborg:
>> On Wed, Apr 26, 2023 at 03:04:15PM +0200, Thomas Zimmermann wrote:
>>> Fbdev provides helpers for framebuffer I/O, such as fb_readl(),
>>> fb_writel() or fb_memcpy_to_fb(). The implementation of each helper
>>> depends on the architecture. It's still all located in fbdev's main
>>> header file <linux/fb.h>. Move all of it into each archtecture's
>>> <asm/fb.h>, with shared code in <asm-generic/fb.h>.
>> 
>> For once I think this cleanup is moving things in the wrong direction.
>> 
>> The fb_* helpers predates the generic io.h support and try to
>> add a generic layer for read read / write operations.
>> 
>> The right fix would be to migrate fb_* to use the io helpers
>> we have today - so we use the existing way to handle the architecture
>> specific details.
>
> I looked through the existing versions of the fb_() I/O helpers. They 
> can apparently be implemented with the regular helpers of similar names.
>
> I'm not sure, but even Sparc looks compatible. At least these sbus_ 
> functions seem to be equivalent to the __raw_() I/O helpers of similar 
> names. Do you still have that Sparc emulator?

I looked at the current code and came to the same conclusion: all
architectures we support today do the same thing in __raw_readl()
and fb_readl() etc, so we can completely remove the latter without
changing semantics.

I think the original list was necessary since not all architectures
supported the __raw_ accessors in the past, so they were open-coded
here for the rest. I thought there were also architectures on which
__raw_readl() does a byteswap to reverse the swap done in a PCI
host bridge, but it apears that none of those remain now, if we ever
had them.

     Arnd