linux-kernel - Re: [PATCH v5 2/3] dma: replace zone_dma_bits by zone_dma

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Message-ID: <20240807161938.5729b656@mordecai.tesarici.cz>
Date: Wed, 7 Aug 2024 16:19:38 +0200
From: Petr Tesařík <petr@...arici.cz>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Baruch Siach <baruch@...s.co.il>, Christoph Hellwig <hch@....de>, Marek
 Szyprowski <m.szyprowski@...sung.com>, Will Deacon <will@...nel.org>, Robin
 Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org, Ramon Fried
 <ramon@...reality.ai>, Elad Nachman <enachman@...vell.com>
Subject: Re: [PATCH v5 2/3] dma: replace zone_dma_bits by zone_dma_limit

On Fri, 2 Aug 2024 10:37:38 +0100
Catalin Marinas <catalin.marinas@....com> wrote:

> On Fri, Aug 02, 2024 at 09:03:47AM +0300, Baruch Siach wrote:
> > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> > index 3b4be4ca3b08..62b36fda44c9 100644
> > --- a/kernel/dma/direct.c
> > +++ b/kernel/dma/direct.c
> > @@ -20,7 +20,7 @@
> >   * it for entirely different regions. In that case the arch code needs to
> >   * override the variable below for dma-direct to work properly.
> >   */
> > -unsigned int zone_dma_bits __ro_after_init = 24;
> > +u64 zone_dma_limit __ro_after_init = DMA_BIT_MASK(24);  
> 
> u64 here makes sense even if it may be larger than phys_addr_t. It
> matches the phys_limit type in the swiotlb code. The compilers should no
> longer complain.

FTR I have never quite understood why phys_limit is u64, but u64 was
already used all around the place when I first looked into swiotlb.

> > diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
> > index d10613eb0f63..7b04f7575796 100644
> > --- a/kernel/dma/pool.c
> > +++ b/kernel/dma/pool.c
> > @@ -70,9 +70,9 @@ static bool cma_in_zone(gfp_t gfp)
> >  	/* CMA can't cross zone boundaries, see cma_activate_area() */
> >  	end = cma_get_base(cma) + size - 1;
> >  	if (IS_ENABLED(CONFIG_ZONE_DMA) && (gfp & GFP_DMA))
> > -		return end <= DMA_BIT_MASK(zone_dma_bits);
> > +		return end <= zone_dma_limit;
> >  	if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32))
> > -		return end <= DMA_BIT_MASK(32);
> > +		return end <= max(DMA_BIT_MASK(32), zone_dma_limit);
> >  	return true;
> >  }
> >  
> > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > index 043b0ecd3e8d..bb51bd5335ad 100644
> > --- a/kernel/dma/swiotlb.c
> > +++ b/kernel/dma/swiotlb.c
> > @@ -450,9 +450,9 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
> >  	if (!remap)
> >  		io_tlb_default_mem.can_grow = true;
> >  	if (IS_ENABLED(CONFIG_ZONE_DMA) && (gfp_mask & __GFP_DMA))
> > -		io_tlb_default_mem.phys_limit = DMA_BIT_MASK(zone_dma_bits);
> > +		io_tlb_default_mem.phys_limit = zone_dma_limit;
> >  	else if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp_mask & __GFP_DMA32))
> > -		io_tlb_default_mem.phys_limit = DMA_BIT_MASK(32);
> > +		io_tlb_default_mem.phys_limit = max(DMA_BIT_MASK(32), zone_dma_limit);
> >  	else
> >  		io_tlb_default_mem.phys_limit = virt_to_phys(high_memory - 1);
> >  #endif  
> 
> These two look correct to me now and it's the least intrusive (the
> alternative would have been a zone_dma32_limit). The arch code, however,
> needs to ensure that zone_dma_limit can always support 32-bit devices
> even if it is above 4GB (with the relevant dma offsets in place for such
> devices).

Just to make sure, the DMA zone (if present) must map to at most 32-bit
bus address space (possibly behind a bridge). Is that what you're
saying?

Petr T