BAT mapping exported to user-space

[Linh Dang]

Hi all,

In our project, our application continuously does random-access to
various ASICs. The ASICs are mapped by a bat register:

- Forcing the application to use system-calls for accessing the ASICs
  would incur a too-big overhead.

- Adding _PAGE_USER to bat-mapping is ok but would export the mapping
  to every single userspace processes.

- I'm contemplating adding 2 bitmasks (1 read, 1 write) into the
  mm_struct:
  - each bit in the mask represent a BAT register that should be
    accessible from userspace.
  - in switch_mm, prior to the call to set_context(), the BATs's Vp
    bits would be updated based on the masks of the `next' mm.

My questions are:
- has it been discussed/comptemplate/done before?
- what issues (performance,style,...) do you think such a change would
  create?
- How the X server access the hw on PPC?


Thanx

--
Linh Dang

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