Date: Wed, 17 Feb 1999 12:20:50 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Kevin Day <toasty@home.dragondata.com> Cc: dyson@iquest.net, tlambert@primenet.com, mike@smith.net.au, hackers@FreeBSD.ORG Subject: Re: vm_page_zero_fill Message-ID: <199902172020.MAA10702@apollo.backplane.com> References: <199902172005.OAA29252@home.dragondata.com>
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:> :logic analyzer is a form of witchcraft... If this is the case, turning this
:> :off would greatly help me. :)
:>
:> It should not be zeroing pages before doing full reads into them.
:> That is pretty well optimized, usually.
:
:What if I'm doing a partial read? Is a partial read even possible if I'm
:using the mmap method?
The filesystem always does a full read even if you only do a partial
read. This is true whether you use read() or access a page w/ mmap(),
but it is doubly true with mmap().
:I tried this, but ran into a few problems.
:
:First, I had to somehow convince the vm system to bring the pages in from
:disk, before doing the DMA, and making sure they were contiguously mapped to
:physical ram, and then forcing it not to dump them later. Never quite got
:past this hurdle. :) Is there a driver somewhere that does this? In my
:system, userland code mmap()'s data, and does memcpy's to a mmaped device
:that corresponds directly to the physical frame buffer. I wasn't really sure
:how to make sure the data ended up in a nice contiguous buffer to DMA it out
:of, without doing another copy.
You would have to design a device to do it. Not terrible, but not
trivial either. But that wouldn't get rid of the memcpy... the only
way to get rid of the memcpy is to make the filesystem write the
data from the file directly to the frame buffer by supplying
ficticious pages directly to the underlying block device.
This would be a difficult hack to do in the filesystem, but you
might be able to do it talking to a raw device and not have to
hack the system at all.
For example, create a partition on the hard disk that you intend
to access directly. i.e. no filesystem.
If you mmap() the frame buffer, and issue a read() on the raw
device interface for the partition with a pointer into the
mmap()'d frame buffer, I *believe* this will result in
direct DISK->PCI DMA. I'm not positive, though.
:> the SCSI bus and have it master the data directly from the
:> drives without host intervention ). This is a rather more
:> complex solution.
:
:We're using IDE too. :)
It is a DMA capable IDE controller, right? If not, you are
S.O.L.
:Thanks,
:
:Kevin
-Matt
Matthew Dillon
<dillon@backplane.com>
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