Re: [PATCH v2 01/19] fbdev: Prepare generic architecture helpers

Hi Geert

Am 11.04.23 um 10:08 schrieb Geert Uytterhoeven:
> Hi Thomas,
> 
> On Thu, Apr 6, 2023 at 4:30 PM Thomas Zimmermann <[email protected]> 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. The default mode set by
>> fb_pgprotect() is now writecombine, which is what most platforms
>> want.
>>
>> 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>.
>>
>> v2:
>>          *  use writecombine mappings by default (Arnd)
>>
>> Signed-off-by: Thomas Zimmermann <[email protected]>
> 
> Thanks for your patch!
> 
>> --- a/include/asm-generic/fb.h
>> +++ b/include/asm-generic/fb.h
>> @@ -1,13 +1,32 @@
>>   /* SPDX-License-Identifier: GPL-2.0 */
>> +
>>   #ifndef __ASM_GENERIC_FB_H_
>>   #define __ASM_GENERIC_FB_H_
>> -#include <linux/fb.h>
>>
>> -#define fb_pgprotect(...) do {} while (0)
>> +/*
>> + * Only include this header file from your architecture's <asm/fb.h>.
>> + */
>> +
>> +#include <asm/page.h>
>> +
>> +struct fb_info;
>> +struct file;
>> +
>> +#ifndef fb_pgprotect
>> +#define fb_pgprotect fb_pgprotect
>> +static inline void fb_pgprotect(struct file *file, struct vm_area_struct *vma,
>> +                               unsigned long off)
> 
> Does this affect any noMMU platforms that relied on fb_pgprotect()
> doing nothing before?
> Perhaps the body below should be protected by "#ifdef CONFIG_MMU"?

I cannot conclusively answer this question, but I did some grep'ing 
('git grep ndef | grep CONFIG_MMU'):

Only the architectures in this patchset provide <asm/fb.h> but nothing 
anywhere uses <asm-generic/fb.h> yet. And of those architectures, only 
arm and m68k have !CONFIG_MMU cases. Those are handled in the rsp 
patches. I think we're good.

> 
>> +{
>> +       vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> 
> Shouldn't this use the pgprot_val() wrapper?

No, I think. Grep'ing for vm_page_prot, I'm not seeing it being used in 
such assignments.

Best regards
Thomas

> 
>> +}
>> +#endif
> 
> Gr{oetje,eeting}s,
> 
>                          Geert
> 

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