Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Stefano Stabellini]

On Fri, 19 Mar 2021, Konrad Rzeszutek Wilk wrote:
> On Fri, Mar 19, 2021 at 02:07:31PM +0100, Christoph Hellwig wrote:
> > On Thu, Mar 18, 2021 at 09:03:33PM -0700, Florian Fainelli wrote:
> > >  #ifdef CONFIG_ARM_LPAE
> > > +	if (swiotlb_force == SWIOTLB_FORCE ||
> > > +	    max_pfn > arm_dma_pfn_limit)
> > 
> > Does arm_dma_pfn_limit do the right thing even with the weirdest
> > remapping ranges?  Maybe a commen here would be useful.
> > 
> > > +		swiotlb_init(1);
> > > +	else
> > > +		swiotlb_force = SWIOTLB_NO_FORCE;
> > 
> > Konrad: what do you think of setting swiotlb_force to SWIOTLB_NO_FORCE
> > and only switching it to SWIOTLB_NORMAL when swiotlb_init* is called?
> > That kind makes more sense than forcing the callers to do it.
> > 
> > While we're at it, I think swiotlb_force should probably be renamed to
> > swiotlb_mode or somethng like that.
> 
> swiotlb_mode sounds good.
> 
> Also it got me thinking - ARM on Xen at some point was a bit strange, so not sure how
> the logic works here, Stefano?

There is nothing strange in regards to swiotlb_force. swiotlb_force is only used
in swiotlb-xen map_page to figure out whether:

- we actually have to use the swiotlb bounce buffer (this is the
  swiotlb_xen == SWIOTLB_FORCE case)
- or we can use the provided page directly for dma if other conditions
  are met (dma_capable, !range_straddles_page_boundary, ...)


I don't think that switching to "swiotlb_mode" would cause any issues.