linux-kernel - Re: [PATCH v3 4/6] fbdev: Include <linux/io.h> via <asm/fb.h>

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Message-ID: <20230502195429.GA319489@ravnborg.org>
Date:   Tue, 2 May 2023 21:54:29 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     deller@....de, geert@...ux-m68k.org, javierm@...hat.com,
        daniel@...ll.ch, vgupta@...nel.org, chenhuacai@...nel.org,
        kernel@...0n.name, davem@...emloft.net,
        James.Bottomley@...senpartnership.com, arnd@...db.de,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        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 v3 4/6] fbdev: Include <linux/io.h> via <asm/fb.h>

Hi Thomas,

On Tue, May 02, 2023 at 03:02:21PM +0200, Thomas Zimmermann wrote:
> Fbdev's main header file, <linux/fb.h>, includes <asm/io.h> to get
> declarations for I/O helper functions. From these declarations, it
> later defines framebuffer I/O helpers, such as fb_{read,write}[bwlq]()
> or fb_memset().
> 
> The framebuffer I/O helpers depend on the system architecture and
> will therefore be moved into <asm/fb.h>. Prepare this change by first
> adding an include statement for <linux/io.h> to <asm-generic/fb.h>.
> Include <asm/fb.h> in all source files that use the framebuffer I/O
> helpers, so that they still get the necessary I/O functions.
> 
...
> 
> diff --git a/drivers/video/fbdev/arkfb.c b/drivers/video/fbdev/arkfb.c
> index 60a96fdb5dd8..fd38e8a073b8 100644
> --- a/drivers/video/fbdev/arkfb.c
> +++ b/drivers/video/fbdev/arkfb.c
> @@ -27,6 +27,8 @@
>  #include <linux/console.h> /* Why should fb driver call console functions? because console_lock() */
>  #include <video/vga.h>
>  
> +#include <asm/fb.h>

When we have a header like linux/fb.h - it is my understanding that it is
preferred to include that file, and not the asm/fb.h variant.

This is assuming the linux/fb.h contains the generic stuff, and includes
asm/fb.h for the architecture specific parts.

So drivers will include linux/fb.h and then they automatically get the
architecture specific parts from asm/fb.h.

In other words, drivers are not supposed to include asm/fb.h, if
linux.fb.h exists - and linux/fb.h shall include the asm/fb.h.

If the above holds true, then it is wrong and not needed to add asm/fb.h
as seen above.


There are countless examples where the above are not followed,
but to my best understanding the above it the preferred way to do it.

	Sam