Framebuffer mmap on PowerPC

[Christophe Leroy]

Le 31/08/2023 à 16:41, Thomas Zimmermann a écrit :
> Hi,
> 
> there's a per-architecture function called fb_pgprotect() that sets 
> VMA's vm_page_prot for mmaped framebuffers. Most architectures use a 
> simple implementation based on pgprot_writecomine() [1] or 
> pgprot_noncached(). [2]
> 
> On PPC this function uses phys_mem_access_prot() and therefore requires 
> the mmap call's file struct. [3] Removing the file argument would help 
> with simplifying the caller of fb_pgprotect(). [4]
> 
> Why is the file even required on PPC?
> 
> Is it possible to replace phys_mem_access_prot() with something simpler 
> that does not use the file struct?

Looks like phys_mem_access_prot() defaults to calling pgprot_noncached() 
see 
https://elixir.bootlin.com/linux/v6.5/source/arch/powerpc/mm/mem.c#L37 
but for a few platforms that's superseeded by 
pci_phys_mem_access_prot(), see 
https://elixir.bootlin.com/linux/v6.5/source/arch/powerpc/kernel/pci-common.c#L524

However, as far as I can see pci_phys_mem_access_prot() doesn't use file 
so you could likely drop that argument on phys_mem_access_prot() on 
powerpc. But when I for instance look at arm, I see that the file 
argument is used, see 
https://elixir.bootlin.com/linux/v6.5/source/arch/arm/mm/mmu.c#L713

So, the simplest is maybe the following, allthough that's probably worth 
a comment:

diff --git a/arch/powerpc/include/asm/fb.h b/arch/powerpc/include/asm/fb.h
index 5f1a2e5f7654..8b9b856f476e 100644
--- a/arch/powerpc/include/asm/fb.h
+++ b/arch/powerpc/include/asm/fb.h
@@ -6,10 +6,9 @@

  #include <asm/page.h>

-static inline void fb_pgprotect(struct file *file, struct 
vm_area_struct *vma,
-				unsigned long off)
+static inline void fb_pgprotect(struct vm_area_struct *vma, unsigned 
long off)
  {
-	vma->vm_page_prot = phys_mem_access_prot(file, off >> PAGE_SHIFT,
+	vma->vm_page_prot = phys_mem_access_prot(NULL, off >> PAGE_SHIFT,
  						 vma->vm_end - vma->vm_start,
  						 vma->vm_page_prot);
  }


Christophe


> 
> Best regards
> Thomas
> 
> 
> [1] 
> https://elixir.bootlin.com/linux/v6.5/source/include/asm-generic/fb.h#L19
> [2] 
> https://elixir.bootlin.com/linux/v6.5/source/arch/mips/include/asm/fb.h#L11
> [3] 
> https://elixir.bootlin.com/linux/v6.5/source/arch/powerpc/include/asm/fb.h#L12
> [4] 
> https://elixir.bootlin.com/linux/v6.5/source/drivers/video/fbdev/core/fbmem.c#L1299
> 
>