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To: "Stan Shkolny" <stan@osgroup.com>
Cc: hackers@FreeBSD.ORG
Subject: Re: vmapbuf (was:Which O/S routines are subject to change?) 
In-reply-to: Your message of "Tue, 11 May 1999 10:06:00 CDT."
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Date: Sun, 16 May 1999 13:05:05 -0700
From: Mike Smith <mike@smith.net.au>
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> My character device driver receives a pointer to a user data buffer in the
> user's address space. I need to DMA those data. For this, I need to bring
> the pages into memory and lock them before initiating the DMA.
> 
> My book tells about physio(). However, I found it unconvenient for me.
> One reason is that its functionality doesn't fit well into my driver's
> architecture. 

You should architect your driver to suit the way the kernel wants to do 
things; that's generally the role of a driver, to shim between the 
kernel and the hardware.  If you find your driver architecture isn't 
fitting the kernel well, you need to fix your driver.

> In other words, I want to write my own routine.
> 
> In general, physio() looks like this:
> 
> if useracc()
>     vmapbuf()
>     *strategy()
>     vunmapbuf()
> endif
> 
> And vmapbuf() like this:
> 
> vm_fault_quick()
> vm_page_hold()
> pmap_kenter()    // maps the memory into the kernel space
> 
> And I want to write something like this in my driver:
> if useracc()
>     vm_fault_quick()
>     vm_page_hold()
>     <initiating DMA>
> endif
> 
> So, is this likely for the above 3 functions to change or disappear in the
> next versions? Can they be considered "stable"?

No, I don't think so.  physio() is the "exported" interface for this 
sort of thing.  If there are performance defects with using physio they 
should be fixed there.  All your custom interface wins you is not 
mapping the buffer into the kernel space, but this will actually hurt 
you down the track when you try to do the DMA, as it's much harder to 
traslate the user virtual address into a physical address when you do 
the DMA setup.

-- 
\\  Sometimes you're ahead,       \\  Mike Smith
\\  sometimes you're behind.      \\  mike@smith.net.au
\\  The race is long, and in the  \\  msmith@freebsd.org
\\  end it's only with yourself.  \\  msmith@cdrom.com




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