From owner-freebsd-hackers  Tue May  9  2:58:53 2000
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To: Peter Edwards <peter.edwards@openet-telecom.com>
Cc: hackers@FreeBSD.ORG, Coleman Kane <cokane@one.net>
Subject: Re: mmap cdev function in device drivers 
In-reply-to: Your message of "Tue, 09 May 2000 10:55:19 BST."
             <3917E087.B2477FA0@openet-telecom.com> 
Date: Tue, 09 May 2000 11:58:04 +0200
Message-ID: <24302.957866284@critter.freebsd.dk>
From: Poul-Henning Kamp <phk@critter.freebsd.dk>
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A good example to look at is the pci/xrpu.c file, that driver
barely does anything but a mmap.

Poul-Henning

In message <3917E087.B2477FA0@openet-telecom.com>, Peter Edwards writes:
>Hi,
>Just trying to take some of the aforementioned "magic" out of i386_btop
>/ vtop :-)
>
>>    return( atop(vtophys(bktr->bigbuf) + offset) );
>
>atop  (I assume) stands for "address to page" (given a pointer, give the
>number of the page it is in)
>vtophys is "virtual to physical". (given a pointer in a virtual address
>space, find out the physical address of the backing memory.)
>
>My understanding is that mmap(2) will allocate a portion of the calling
>process's address space, and for each page it needs to map, will call
>the device's mmap function, giving it the calculated offset (and the
>protection attributes).
>
>The device's mmap returns the index of the physical page of the memory
>to be inserted under the virtual addresses the process sees.
>
>simplified_mmap_syscall_for_device(dev_t device, size_t len, off_t
>offset)
>{
>	caddr_t ptr = alloc_address_space(len);
>
>	assert(ptr % PAGESIZE == 0);
>
>	while (len) {
>		pageno = device->mmap(offset); /* Call device's mmap */
>		map_address_to_page(ptr, pageno);
>		len -= PAGESIZE;
>		offset += PAGESIZE;
>		ptr += PAGESIZE;
>	}
>}
>
>So, the call above is returning the page number (of the physical address
>(of bktr->bigbuf)).
>
>Of course, My ignorance will probably be corrected in due course!
>--
>Peter.
>
>
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--
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD coreteam member | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.


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