port-amd64/57835: x86 bus_dmamap_load_raw allegedly fails to respect maxsegsz

[campbell+netbsd%mumble.net@localhost]

>Number:         57835
>Category:       port-amd64
>Synopsis:       x86 bus_dmamap_load_raw allegedly fails to respect maxsegsz
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Jan 11 04:20:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current
>Organization:
The NetBSD Gemdation
>Environment:
>Description:
In i915_gem_stolen.c, I wrote this comment some years ago, presumably on the basis of some empirical drudgery:

	/*
	 * XXX x86 bus_dmamap_load_raw fails to respect the maxsegsz we
	 * pass to bus_dmamap_create, so we have to create page-sized
	 * segments to begin with.
	 */

https://nxr.netbsd.org/xref/src/sys/external/bsd/drm2/dist/drm/i915/gem/i915_gem_stolen.c?r=1.5#540

I haven't looked recently at x86 bus_dmamap_load_raw, so I don't know if this is true or up-to-date.

We should either

(a) find the bug and fix it, or
(b) determine that x86 bus_dmamap_load_raw does respect maxsegsz after all and delete the comments and workarounds.
>How-To-Repeat:
code inspection
>Fix:
Yes, please!