On Mon, 2017-04-03 at 15:52 +0100, Robin Murphy wrote: > On 01/04/17 08:25, Scott Wood wrote: > > > > On Fri, 2017-03-31 at 18:55 +0100, Robin Murphy wrote: > > > > > > On 31/03/17 04:27, Michael Ellerman wrote: > > > > > > > > > > > > Robin Murphy <robin.murphy@xxxxxxx> writes: > > > > > > > > > > > > > > > > > > > Hi Roy, > > > > > > > > > > On 29/03/17 22:13, Roy Pledge wrote: > > > > > > > > > > > > > > > > > > Use the shared-memory-pool mechanism for frame queue descriptor > > > > > > and > > > > > > packed frame descriptor record area allocations. > > > > > Thanks for persevering with this - in my opinion it's now looking > > > > > like > > > > > it was worth the effort :) > > > > > > > > > > AFAICS the ioremap_wc() that this leads to does appear to give back > > > > > something non-cacheable on PPC (assuming "pgprot_noncached_wc" isn't > > > > > horrendously misnamed), and "no-map" should rule out any cacheable > > > > > linear map alias existing, so it would seem that this approach > > > > > should > > > > > avert Scott's concerns about attribute mismatches. > > > > How does 'no-map' translate into something being excluded from the > > > > linear mapping? > > > Reserved regions marked with "no-map" get memblock_remove()d by > > > early_init_dt_alloc_reserved_memory_arch(). As I understand things, the > > > linear map should only cover memblock areas, and it would be explicitly > > > violating the semantics of "no-map" to still cover such a region. > > Discontiguous memory isn't supported on these PPC chips. Everything up to > > memblock_end_of_DRAM() gets mapped -- and if that were to change, the > > fragmentation would waste TLB1 entries. > Ah, so the "PPC-specific angles I'm not aware of" category is indeed > non-empty - I guess the lack of HAVE_GENERIC_DMA_COHERENT might be > related, then. > > That said, though, AFAICS only certain x86 and s390 configurations ever > call memblock_set_bottom_up(true), so we should be able to assume that > the reserved region allocations always fall through to > __memblock_find_range_top_down(). Thus if your DRAM is contiguous, then > "no-map"-ing the reserved regions will simply end up pushing > memblock_end_of_DRAM() down in a manner that would appear to still avoid > overlaps. Can you guarantee it will be at the end? What if there were other early memblock allocations (e.g. other reserved-memory regions without no-map) that came first? > I can only see that going wrong if the end of DRAM wasn't at > least 32MB aligned to begin with - is that ever likely to happen in > practice? Probably not, unless the user asks for an unusual memory size via mem= or similar. -Scott -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
