On Fri, Oct 1, 2021 at 4:59 AM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote: > > Hi Olof, > > On Fri, Oct 1, 2021 at 7:36 AM Olof Johansson <olof@xxxxxxxxx> wrote: > > A much more valuable approach would be to work towards being able to > > free up memory by un-probed drivers at the end of boot. That would > > possibly benefit all platforms on all architectures. > > We used to have such a functionality in arch/ppc (not arch/powerpc!), > where code/data could be tagged __prep, __chrp, or __pmac, to put it > in a special section, and to be freed with initdata when unused. It > was removed in v2.6.15[1], as the savings weren't worth the hassle. > In a more fragmented space like arm the memory lost due to alignment > of the sections would be even more substantial. Yeah, the balance between per-platform code size and overall kernel code size shifted over time to a point where it wasn't as meaningful on ppc. > Another problem is to know when is the end of the boot, especially > with deferred probing. Most of this code either has a module_init() or an initcall that actually registers the drivers and/or probes for the platform and does the work. This means you can have a late equivalent hook/initcall that determines whether this path ended up being probed/used. If it wasn't, you can then unregister and flag the corresponding memory to be freed at the end, and would take out the heuristics and guessing on needing to do it automatically from the code path that's doing said freeing. -Olof
