[PATCH v5 2/3] dma: replace zone_dma_bits by zone_dma_limit

[Robin Murphy]

On 2024-08-08 10:35 am, Petr Tesařík wrote:
> On Wed, 7 Aug 2024 19:14:58 +0100
> Catalin Marinas <catalin.marinas at arm.com> 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 at arm.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_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.

Right, that's why the *_dma_get_max_cpu_address() functions already walk 
all known bus translations backwards to find the lowest common 
denominator in the CPU address space. In principle we could also 
calculate the lowest translated 32-bit DMA address from every >32-bit 
range in the same way, however that represents enough extra complexity 
that it doesn't seem worth trying to implement unless and until someone 
actually has a clear need for it.

Thanks,
Robin.

> 
> 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