Messages in this thread | | | Subject | Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB | From | Robin Murphy <> | Date | Thu, 18 Mar 2021 23:35:00 +0000 |
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On 2021-03-18 21:31, Florian Fainelli wrote: > > > 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.
Eh? When did 2KB become 64MB? IO_TLB_SHIFT is 11, so that's at most one page in anyone's money...
>>>> 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@gmail.com> >>>>> >>>>> 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.
In fact I should have looked more closely at that myself - checking debugfs on my 4GB arm64 board actually shows io_tlb_nslabs = 0, and indeed we are bypassing initialisation completely and (ab)using SWIOTLB_NO_FORCE to cover it up, so I guess it probably *is* safe now for the noforce option to do the same for itself and save even that one page.
Robin.
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