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

[Robin Murphy]

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.

> > 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.