On Wed, 7 Aug 2024 19:14:58 +0100 Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > On Wed, Aug 07, 2024 at 04:19:38PM +0200, Petr Tesařík wrote: > > On Fri, 2 Aug 2024 10:37:38 +0100 > > Catalin Marinas <catalin.marinas@xxxxxxx> 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_mhttps://lpc.events/event/18/contributions/1776/ask & __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? > > No exactly. What I'm trying to say is that on arm64 zone_dma_limit can > go beyond DMA_BIT_MASK(32) when the latter is treated as a CPU address. > In such cases, ZONE_DMA32 is empty. > > TBH, this code is confusing and not entirely suitable for a system where > the CPU address offsets are not 0. The device::dma_coherent_mask is > about the bus address range and phys_limit is calculated correctly in > functions like dma_direct_optimal_gfp_mask(). But that's about it w.r.t. > DMA bit masks because zone_dma_bits and DMA_BIT_MASK(32) are assumed to > be about the CPU address ranges in some cases (in other cases > DMA_BIT_MASK() is used to initialise dma_coherent_mask, so more of a bus > address). Yes, I know. > On the platform Baruch is trying to fix, RAM starts at 32GB and ZONE_DMA > should end at 33GB. That's 30-bit mask in bus address terms but > something not a power of two for the CPU address, hence the > zone_dma_limit introduced here. Yes, I was watching the discussion. > With ZONE_DMA32, since all the DMA code assumes that ZONE_DMA32 ends at > 4GB CPU address, it doesn't really work for such platforms. If there are > 32-bit devices with a corresponding CPU address offset, ZONE_DMA32 > should end at 36GB on Baruch's platform. But to simplify things, we just > ignore this on arm64 and make ZONE_DMA32 empty. Ah. That makes sense. It also seems to support my theory that Linux memory zones are an obsolete concept and should be replaced by a different mechanism. > In some cases where we have the device structure we could instead do a > dma_to_phys(DMA_BIT_MASK(32)) but not in the two cases above. I guess if > we really want to address this properly, we'd need to introduce a > zone_dma32_limit that's initialised by the arch code. For arm64, I'm > happy with just having an empty ZONE_DMA32 on such platforms. The obvious caveat is that zone boundaries are system-wide, but the mapping between bus addresses and CPU addresses depends on the device structure. After all, that's why dma_to_phys takes the device as a parameter... In fact, a system may have multiple busses behind different bridges with a different offset applied by each. FYI I want to make more people aware of these issues at this year's Plumbers, see https://lpc.events/event/18/contributions/1776/ Petr T
