[haiku-bugs] Re: [Haiku] #8514: allow mmap()ing devices
- From: "bonefish" <trac@xxxxxxxxxxxx>
- Date: Wed, 02 May 2012 19:46:24 -0000
#8514: allow mmap()ing devices
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Reporter: pulkomandy | Owner: pulkomandy
Type: enhancement | Status: new
Priority: low | Milestone: Unscheduled
Component: Drivers | Version: R1/Development
Resolution: | Keywords:
Blocked By: | Blocking:
Has a Patch: 0 | Platform: All
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Comment (by bonefish):
Mapping the frame buffer is a bit of a special case. It really is physical
memory and there are already interfaces to map it into an application
address space (`BWindowScreen`/`BDirectWindow`). It probably wouldn't be
hard to map it also via `mmap()`, but I don't really see the point.
Regarding disk devices, that is more interesting (and involved) and has
already been discussed, though from a different angle. One problem Haiku's
kernel currently has is the block cache. It uses the slab to store block
data, which uses (virtual) kernel address space. Kernel address space,
however, is relatively limited on a 32 bit system. Moreover, using the
slab while the file cache uses physical memory causes an imbalance in
favor of the block cache.
What has been discussed is a unification of the cache architecture. Like
the file cache, the block cache would use physical memory directly
(organized in a `VMCache` object). This would also allow to easily layer
caches, since `VMCache` already supports such a mechanism. So one could
have a cache for the whole device and caches on top of it for the
partitions which would replace the current block caches. The device cache
would be located at the I/O scheduler level -- either integrated or on top
of it (in fact for CDs/DVDs a similar thing does already exist: `IOCache`,
a special `IOScheduler`, already uses a `VMCache` to cache the disk
contents).
As a side effect the devices (and partitions) can easily be made
`mmap()`able. The reason is that an area just maps a `VMCache` to an
address range in some address space.
Long story short, this is all very nice and desirable. Unfortunately it is
also quite a bit of work and has several details that need to be worked
out. Like how to implement transaction support (which the block cache
requires), how to deal with block size != page size, etc. It would
probably be a good idea to kick start such a development during a coding
sprint, so that two or three people can work on it together.
--
Ticket URL: <http://dev.haiku-os.org/ticket/8514#comment:1>
Haiku <http://dev.haiku-os.org>
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