Re: Framebuffer mmap on PowerPC

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Hi

Am 31.08.23 um 19:38 schrieb 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

Right, I've seen these various implementations. Luckily, the ARM framebuffers use a plain pgprot_writecombine() without any references on to file.


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

Could we drop the file argument from PPC's internal functions and provide this interface to fb_pgprotect()? phys_mem_access_prot() would be a trivial wrapper around that internal API. I'd provide a patch to do that.

Best regards
Thomas


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



--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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