Re: [PATCH RFC 3/4] dma-direct: add offset to zone_dma_bits

[Baruch Siach]

Hi Catalin,

On Mon, Jan 08 2024, Catalin Marinas wrote:
> On Wed, Dec 27, 2023 at 05:04:27PM +0200, Baruch Siach wrote:
>> Current code using zone_dma_bits assume that all addresses range in the
>> bits mask are suitable for DMA. For some existing platforms this
>> assumption is not correct. DMA range might have non zero lower limit.
>> 
>> Add 'zone_dma_off' for platform code to set base address for DMA zone.
>> 
>> Rename the dma_direct_supported() local 'min_mask' variable to better
>> describe its use as limit.
>> 
>> Suggested-by: Catalin Marinas <catalin.marinas@xxxxxxx>
>
> When I suggested taking the DMA offsets into account, that's not exactly
> what I meant. Based on patch 4, it looks like zone_dma_off is equivalent
> to the lower CPU address. Let's say a system has DRAM starting at 2GB
> and all 32-bit DMA-capable devices has a DMA offset of 0. We want
> ZONE_DMA32 to end at 4GB rather than 6GB.

Patch 4 sets zone_dma_off to the lower limit from 'dma-ranges' property
that determines zone_dma_bits. This is not necessarily equivalent to
start of DRAM, though it happens to be that way on my platform.

>> @@ -59,7 +60,7 @@ static gfp_t dma_direct_optimal_gfp_mask(struct device *dev, u64 *phys_limit)
>>  	 * zones.
>>  	 */
>>  	*phys_limit = dma_to_phys(dev, dma_limit);
>> -	if (*phys_limit <= DMA_BIT_MASK(zone_dma_bits))
>> +	if (*phys_limit <= zone_dma_off + DMA_BIT_MASK(zone_dma_bits))
>>  		return GFP_DMA;
>>  	if (*phys_limit <= DMA_BIT_MASK(32))
>>  		return GFP_DMA32;
>
> Ah, you ignore the zone_dma_off for 32-bit calculations. But the
> argument still stands, the start of DRAM does not necessarily mean that
> all non-64-bit devices have such DMA offset.
>
> The current dma_direct_optimal_gfp_mask() confuses me a bit, I think it
> gives the wrong flag if we have a zone_dma_bits of 30 and a device with
> a coherent_dma_mask of 31, it incorrectly ends up with GFP_DMA32 (I'm
> ignoring dma offsets in this example). Luckily I don't think we have any
> set up where this would fail. Basically if *phys_limit is strictly
> smaller than DMA_BIT_MASK(32), we want GFP_DMA rather than GFP_DMA32
> even if it is larger than DMA_BIT_MASK(zone_dma_bits).
>
> Anyway, current mainline assumes that DMA_BIT_MASK(zone_dma_bits) and
> DMA_BIT_MASK(32) are CPU addresses. The problem is that we may have the
> start of RAM well above 4GB and neither ZONE_DMA nor ZONE_DMA32 upper
> limits would be a power-of-two. We could change the DMA_BIT_MASK(...) to
> be DMA address limits and we end up with something like:
>
> static gfp_t dma_direct_optimal_gfp_mask(struct device *dev, u64 *phys_limit)
> {
> 	u64 dma_limit = min_not_zero(
> 		dev->coherent_dma_mask,
> 		dev->bus_dma_limit);
> 	u64 dma32_limit = dma_to_phys(dev, DMA_BIT_MASK(32));
>
> 	*phys_limit = dma_to_phys(dev, dma_limit);
> 	if (*phys_limit > dma_limit)
> 		return 0;
> 	if (*phys_limit = dma32_limit)
> 		return GFP_DMA32;
> 	return GFP_DMA;
> }
>
> The alternative is to get rid of the *_bits variants and go for
> zone_dma_limit and zone_dma32_limit in the generic code. For most
> architectures they would match the current DMA_BIT_MASK(32) etc. but
> arm64 would be able to set some higher values.
>
> My preference would be to go for zone_dma{,32}_limit, it's easier to
> change all the places where DMA_BIT_MASK({zone_dma_bits,32}) is used.

Sounds good to me.

Thanks for your review of this confusing piece of code.

baruch

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