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    SubjectRe: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB
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    On 3/18/2021 4:35 PM, Robin Murphy wrote:
    > 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...

    Yes, sorry incorrect shift applied here. Still, and I believe this is
    what you mean below, architecture code setting swiotlb_force =
    SWIOTLB_NO_FORCE does not result in not allocating the SWIOTLB, because
    io_tlb_nslabs is still left set to 0 so swiotlb_init() will proceed with
    allocating the default size.

    >
    >>>>> 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.

    OK, I can submit a patch that does that. 5.12-rc3 works correctly for me
    here as well and only allocates SWIOTLB when needed which in our case is
    either:

    - we have DRAM at PA >= 4GB
    - we have limited peripherals (Raspberry Pi 4 derivative) that can only
    address the lower 1GB

    Now let's see if we can get ARM 32-bit to match :)
    --
    Florian

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