$NetBSD: TODO,v 1.2 2002/09/28 13:06:49 scw Exp $

TODO List For NetBSD/sh5
~~~~~~~~~~~~~~~~~~~~~~~~

In no particular order:


1.	Enable the cpu cache.	*MOSTLY DONE*
	There are two cache modes available in the current cpu: write-back
	and write-through. The former would be very nice. However, I don't
	want to enable caching until I'm happy enough with the stability
	of the pmap. And anyway, the pmap doesn't pay lip-service to the
	cache yet.

2.	Re-work the TLB invalidation code.
	This is a pain, as there is no hardware support for looking up an
	address in the TLBs. To get around this, the PTEG structure
	used to track user-space mappings contains a hint as to which group
	of four TLB slots a mapping is entered on. This information is
	maintained and updated by the TLB miss code itself. Higher level
	pmap code uses the hint to speed up TLB invalidation for user-space
	mappings (a maximum of 4 ITLB and/or 4 DTLB slots need be searched).

	Unfortunately, no such "hint" is maintained for kernel mappings
	(those in KSEG1), making KSEG1 TLB invalidations very expensive.

	Adding a "hint" field to the pmap_kernel_ipt structure would make
	things much quicker, but would use more memory... It's a question
	of trade offs.

	When I make the above change, I'd like to add a similar "hint"
	field to the PTEG structure, instead of relying on "unused" bits
	in the PTEH. This will allow the "hint" to pinpoint *exactly*
	which TLB slot in both ITLB and DTLB contains the mapping. This
	would work for both user-space and KSEG1 mappings.

3.	Allocate interrupt handles from a pool backed by pages from KSEG0.
	This will reduce DTLB misses at interrupt time.

4.	copy{in,out}() need to be re-written.
	Right now, they copy byte-at-a-time, which is not exactly fast...

5.	in_cksum.c needs to be replaced with an assembly code version
	which makes use of special SHmedia instructions. At first glance,
	an algorithm which makes use of the M* instructions would be
	very fast indeed.

6.	libkern needs sh5-specific optimised versions of key functions.
	Most of these can be used by libc too.

7.	Support for more of the on-chip peripherals, such as the DMAC.

8.	Test the whole lot using a 64-bit kernel/userland.

9.	If (8) works, try 64-bit kernel and 32-bit userland, using COMPAT32.

10.	Stress-testing of just about everything. ;-)