Re: drivers customizing mmap(2)

[David Young <dyoung%pobox.com@localhost>]

On Tue, Mar 05, 2013 at 12:31:37AM +0000, Taylor R Campbell wrote:
> I'd like to add a member fo_mmap to struct fileops so that cloning
> devices can support mmap and use other kinds of uvm objects than just
> uvm_deviceops and uvm_vnodeops.
> 
> Currently the only ways to customize mmap(2) are
> 
> . with a device node, by setting d_mmap in the struct cdevsw, but the
> customization is very limited -- you have to use uvm_deviceops, and
> you can't have per-open state for the device (e.g., with a cloning
> device -- file descriptors for a cloning device have type DTYPE_MISC
> which mmap(2) rejects); or
> 
> . with a file system, by providing VOP_MMAP, but you're limited to
> uvm_vnodeops, and a file system is a complex beast.
> 
> Our drm code currently uses a non-cloning device for the drm(4) device
> with some horrible bookkeeping kludges to associate per-open (really,
> per-pid) state with the device.  I'd like to get rid of these horrible
> bookkeeping kludges by using a cloning device, and the newer
> incarnation of drm looks like it will require more of mmap(2) than
> uvm_deviceops and uvm_vnodeops can straightforwardly provide,
> including custom fault handlers.
> 
> I haven't fleshed out a detailed proposal or a patch to implement this
> yet, but it should be fairly straightforward, and may help to clean up
> some of the complex nests of conditionals in uvm_mmap.c.  Thoughts?

At $DAYJOB we wanted an mmap-able character device both to manage the
VA->PA mapping protection (read-only, read-write), to handle faults,
and to remove mappings under certain conditions.  It looked to me like
the uvm_pagerops structure encapsulated all of the functions that our
device needed to supply, but I could find no way for a character device
to supply a custom uvm_pagerops for an mmap'd region.  I thought that
perhaps the device's cdevsw could provide an alternate to d_mmap,
int (*d_get_pagerops)(dev_t, struct uvm_pagerops **) or something,
for uvm_mmap to call.

Dave

-- 
David Young
dyoung%pobox.com@localhost    Urbana, IL    (217) 721-9981