Re: [PATCH] mmc: sdhci-xenon: Fix 2G limitation on AC5 SoC

[Vadym Kochan <vadym.kochan@plvision.eu>]

Hi Robin, Adrian,

On Wed, Aug 17, 2022 at 02:43:46PM +0100, Robin Murphy wrote:
> On 2022-08-16 21:51, Vadym Kochan wrote:
> [...]
> > > The one thing to watch out for is that SWIOTLB doesn't necessarily interact
> > > very well with DMA offsets. Given the intent of
> > > of_dma_get_max_cpu_address(), I think it ought to work out OK now for
> > > current kernels on DT systems if everything is described correctly, but
> > > otherwise it's likely that you end up with ZONE_DMA either being empty or
> > > containing all memory, so the SWIOTLB buffer ends up being allocated
> > > anywhere such that it might not actually work as expected.
> > > 
> > > Robin.
> > 
> > Hi Robin,
> > 
> > Thank you for the reply.
> > 
> > My understanding is that swiotlb is allocated (in case of arm64)
> > in the following cases:
> > 
> >     #1 when it is forced from the kernel cmdline
> > 
> >     #2 when max_pfn is greater than arm64_dma_phys_limit (and this is used
> >        as the end from which to allocate the swiotlb pool in the
> >        top-botom direction via memblock API).
> > 
> >     #3 using restricted dma-pool
> > 
> > Of course option #3 works fine because swiotlb is kind of forced to use
> > particulary this range of memory.
> > 
> > Both options #1 & #2 causes to use full memory mask even if to specify
> > dma-ranges in the DT:
> > 
> >      dma-ranges = <0x0 0x0 0x2 0x0 0x0 0x80000000>;
> > 
> > or if to specify the opposite:
> > 
> >      dma-ranges = <0x2 0x0 0x0 0x0 0x0 0x80000000>;
> > 
> >      just to make it lower than U32 to pass
> > 
> >          zone_dma_bits = min3(32U, dt_zone_dma_bits, acpi_zone_dma_bits)
> > 
> >      condition, but then it will be re-set in max_zone_phys() by:
> > 
> > 	if (phys_start > U32_MAX)
> > 		zone_mask = PHYS_ADDR_MAX;
> > 	else if (phys_start > zone_mask)
> > 		zone_mask = U32_MAX;
> 
> Ah, indeed I missed that, sorry. It seems that that change to stop assuming
> an offset kind of crossed over with the introduction of
> *_dma_get_max_cpu_address(), but now that that firmware property parsing
> *is* implemented, in principle it should be equally possible to evaluate the
> actual offsets as well, and decide whether an offset ZONE_DMA is appropriate
> or not. Either way, this is definitely the area which needs work if we want
> to to able to support topologies like this properly.
> 
> > So, currently I dont see how to pin swiotlb (I see it as a main problem) to some specific range of physical
> > memory (particulary to the first 2G of RAM).
> 
> Indeed, if ZONE_DMA and/or ZONE_DMA32 can't be set appropriately, then
> there's no way to guarantee correct allocation of any DMA buffers, short of
> hacking it with explicitly placed reserved-memory carveouts.
> 

I have sent some time ago a solution which binds restricted-dma pool to
the eMMC device, so Adrian, Robin do you think this can be acceptable as
a temporary solution (at least conceptually) ?

I was also thinking would it be OK to introduce something like
bounced-dma pool (similar to the restricted one) which will reserve
memory for the bounced buffers only ? It should not be hard as looks
like it will re-use existing interface between dma and swiotlb ? In that
case it would allow to map first 2G of memory to eMMC controller.

> Thanks,
> Robin.

Thanks,
Vadym