linux-kernel - Re: Device driver memory 'mmap()' function helper cleanup

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Message-ID: <516E6F4E.9030808@ladisch.de>
Date:	Wed, 17 Apr 2013 11:45:50 +0200
From:	Clemens Ladisch <clemens@...isch.de>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Takashi Iwai <tiwai@...e.de>,
	Mauro Carvalho Chehab <mchehab@...hat.com>
Subject: Re: Device driver memory 'mmap()' function helper cleanup

Arnd Bergmann wrote:
> On Wednesday 17 April 2013, Linus Torvalds wrote:
>> Anyway, I'm attaching the untested patch to several drivers. Guys,
>> mind taking a look?
>
> I took a look at the hpet_mmap function, which still contains this check:
>
>         if (((vma->vm_end - vma->vm_start) != PAGE_SIZE) || vma->vm_pgoff)
>                 return -EINVAL;
>
> As far as I can tell, this check is implied by the new code in
> vm_iomap_memory as the len argument passed here is PAGE_SIZE, so you
> can remove another three lines in hpet_mmap.

Yes indeed.

> > [...] I just have this untested patch-collection for a few random
> > drivers (picked across a few different driver subsystems, just to make
> > it interesting).  I did that largely just to check the different use
> > cases, but I don't actually tend to *use* all that many fancy drivers,
> > so I don't have much of a way of testing it.

Any more-or-less recent x86 machine has HPET, so you could enable
CONFIG_HPET(_MMAP) and try the (completely untested) program below.


Regards,
Clemens

--8<---------------------------------------------------------------->8--

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>

int main(void)
{
	int fd = open("/dev/hpet", O_RDONLY);
	if (fd == -1) {
		perror("/dev/hpet");
		return 1;
	}
	const volatile unsigned int *ptr = mmap(NULL, 4096, PROT_READ, MAP_SHARED, fd, 0);
	if (ptr == MAP_FAILED) {
		perror("mmap");
		return 1;
	}
	printf("frequency: %.5f MHz\n", 1e9 / ptr[1]);
	for (;;) {
		printf("\rcounter: %08x", ptr[60]);
		fflush(stdout);
		usleep(123456);
	}
}
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