linux-kernel - Re: [PATCH] ARM: Qualify enabling of swiotlb

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Date:   Fri, 19 Mar 2021 17:22:28 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
cc:     Christoph Hellwig <hch@....de>, xen-devel@...ts.xensource.com,
        sstabellini@...nel.org, Florian Fainelli <f.fainelli@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        "open list:SWIOTLB SUBSYSTEM" <iommu@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Max Filippov <jcmvbkbc@...il.com>,
        Catalin Marinas <catalin.marinas@....com>, opendmb@...il.com
Subject: Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

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.