Re: [PATCH] arm64: mm: only initialize swiotlb when necessary

[Arnd Bergmann]

On Tuesday, June 21, 2016 5:06:25 PM CEST Catalin Marinas wrote:
> On Wed, Jun 08, 2016 at 05:49:59PM +0200, Arnd Bergmann wrote:
> > On Wednesday, June 8, 2016 1:08:29 PM CEST Catalin Marinas wrote:
> > > On Wed, Jun 08, 2016 at 03:53:46PM +0800, Jisheng Zhang wrote:
> > > >  static int __init arm64_dma_init(void)
> > > >  {
> > > > +     if (swiotlb_force || max_pfn > (arm64_dma_phys_limit >> PAGE_SHIFT))
> > > > +             swiotlb = 1;
> > > > +
> > > >       return atomic_pool_init();
> > > >  }
> > > 
> > > So any platform with RAM larger than 4GB would still initialise swiotlb.
> > > I wouldn't say it's an issue, 64MB is not a significant loss on such
> > > systems.
> > > 
> > > An alternative would be to defer the freeing until we are aware of all
> > > possible dma masks for the populated devices (e.g. from DT), though I'm
> > > not sure that's enough, drivers may try to change such masks when
> > > probed.
> > 
> > Right, this is a hard problem, because you can in theory have odd devices
> > that require a DMA mask lower than the limit of ZONE_DMA.
> 
> I'm not sure what we can do about such devices even with swiotlb. The
> bounce buffer is allocated from ZONE_DMA and currently it assumes a
> 32-bit mask from the start of RAM, so it is not guaranteed to support a
> smaller mask. We may need to come up with some DT memory node attribute
> to define the minimum DMA-able memory and restrict ZONE_DMA during early
> boot but I would rather wait until we hit a real issue in practice.

The bounce buffer is allocated at early boot time in order to get an address
as low as possible, in the hope that it is small enough for those cases.

	Arnd