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

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Date:   Wed, 5 Apr 2023 20:33:50 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Arnd Bergmann <arnd@...db.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, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-sh@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-mips@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, loongarch@...ts.linux.dev,
        sparclinux@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 01/18] fbdev: Prepare generic architecture helpers

Hi

Am 05.04.23 um 17:53 schrieb Arnd Bergmann:
> 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.

Make sense, thanks for the feedback. I'll send out an update soon.

Best regards
Thomas

> 
> 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

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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