On 3/18/2021 12:53 PM, Robin Murphy wrote: > On 2021-03-18 19:43, Florian Fainelli wrote: >> >> >> On 3/18/2021 12:34 PM, Robin Murphy wrote: >>> On 2021-03-18 19:22, Florian Fainelli wrote: >>>> >>>> >>>> On 3/18/2021 12:18 PM, Florian Fainelli wrote: >>>>> It may be useful to disable the SWIOTLB completely for testing or >>>>> when a >>>>> platform is known not to have any DRAM addressing limitations what so >>>>> ever. >>> >>> Isn't that what "swiotlb=noforce" is for? If you're confident that we've >>> really ironed out *all* the awkward corners that used to blow up if >>> various internal bits were left uninitialised, then it would make sense >>> to just tweak the implementation of what we already have. >> >> swiotlb=noforce does prevent dma_direct_map_page() from resorting to the >> swiotlb, however what I am also after is reclaiming these 64MB of >> default SWIOTLB bounce buffering memory because my systems run with >> large amounts of reserved memory into ZONE_MOVABLE and everything in >> ZONE_NORMAL is precious at that point. > > It also forces io_tlb_nslabs to the minimum, so it should be claiming > considerably less than 64MB. IIRC the original proposal *did* skip > initialisation completely, but that turned up the aforementioned issues. AFAICT in that case we will have iotlb_n_slabs will set to 1, which will still make us allocate io_tlb_n_slabs << IO_TLB_SHIFT bytes in swiotlb_init(), which still gives us 64MB. > >>> I wouldn't necessarily disagree with adding "off" as an additional alias >>> for "noforce", though, since it does come across as a bit wacky for >>> general use. >>> >>>>> Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx> >>>> >>>> Christoph, in addition to this change, how would you feel if we >>>> qualified the swiotlb_init() in arch/arm/mm/init.c with a: >>>> >>>> >>>> if (memblock_end_of_DRAM() >= SZ_4G) >>>> swiotlb_init(1) >>> >>> Modulo "swiotlb=force", of course ;) >> >> Indeed, we would need to handle that case as well. Does it sound >> reasonable to do that to you as well? > > I wouldn't like it done to me personally, but for arm64, observe what > mem_init() in arch/arm64/mm/init.c already does. > > Robin. -- Florian
