linux-kernel - Re: [PATCH 01/18] fbdev: Prepare generic architecture helpers

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Date:   Wed, 05 Apr 2023 17:53:03 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Thomas Zimmermann" <tzimmermann@...e.de>,
        "Daniel Vetter" <daniel.vetter@...ll.ch>,
        "Helge Deller" <deller@....de>,
        "Javier Martinez Canillas" <javierm@...hat.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Cc:     Linux-Arch <linux-arch@...r.kernel.org>,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-m68k@...ts.linux-m68k.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 01/18] fbdev: Prepare generic architecture helpers

On Wed, Apr 5, 2023, at 17:05, Thomas Zimmermann wrote:
> Generic implementations of fb_pgprotect() and fb_is_primary_device()
> have been in the source code for a long time. Prepare the header file
> to make use of them.
>
> Improve the code by using an inline function for fb_pgprotect() and
> by removing include statements.
>
> Symbols are protected by preprocessor guards. Architectures that
> provide a symbol need to define a preprocessor token of the same
> name and value. Otherwise the header file will provide a generic
> implementation. This pattern has been taken from <asm/io.h>.
>
> Signed-off-by: Thomas Zimmermann <tzimmermann@...e.de>

Moving this into generic code is good, but I'm not sure
about the default for fb_pgprotect():

> +
> +#ifndef fb_pgprotect
> +#define fb_pgprotect fb_pgprotect
> +static inline void fb_pgprotect(struct file *file, struct vm_area_struct *vma,
> +				unsigned long off)
> +{ }
> +#endif

I think most architectures will want the version we have on
arc, arm, arm64, loongarch, and sh already:

static inline void fb_pgprotect(struct file *file, struct vm_area_struct *vma,
                                unsigned long off)
{
       vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
}

so I'd suggest making that version the default, and treating the
empty ones (m68knommu, sparc32) as architecture specific
workarounds.

I see that sparc64 and parisc use pgprot_uncached here, but as
they don't define a custom pgprot_writecombine, this ends up being
the same, and they can use the above definition as well.

mips defines pgprot_writecombine but uses pgprot_noncached
in fb_pgprotect(), which is probably a mistake and should have
been updated as part of commit 4b050ba7a66c ("MIPS: pgtable.h:
Implement the pgprot_writecombine function for MIPS").

    Arnd