Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

[Robin Murphy]

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@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.

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.