[PATCH][v2] powerpc/mm: using two zones for freescale 64 bit kernel

[Xie Shaohui-B21989]

> -----Original Message-----
> From: Benjamin Herrenschmidt [mailto:benh at kernel.crashing.org]
> Sent: Monday, September 10, 2012 7:38 AM
> To: Kumar Gala
> Cc: Xie Shaohui-B21989; linuxppc-dev at lists.ozlabs.org list; Hu Mingkai-
> B21284; Chen Yuanquan-B41889
> Subject: Re: [PATCH][v2] powerpc/mm: using two zones for freescale 64 bit
> kernel
> 
> On Thu, 2012-08-30 at 15:49 -0500, Kumar Gala wrote:
> > On Aug 24, 2012, at 5:50 AM, Shaohui Xie wrote:
> >
> > > PowerPC platform only supports ZONE_DMA zone for 64bit kernel, so
> > > all the memory will be put into this zone. If the memory size is
> > > greater than the device's DMA capability and device uses
> > > dma_alloc_coherent to allocate memory, it will get an address which
> > > is over the device's DMA addressing, the device will fail.
> > >
> > > So we split the memory to two zones: zone ZONE_DMA32 & ZONE_NORMAL,
> > > since we already allocate PCICSRBAR/PEXCSRBAR right below the 4G
> > > boundary (if the lowest PCI address is above 4G), so we constrain
> > > the DMA zone ZONE_DMA32 to 2GB, also, we clear flag __GFP_DMA &
> > > __GFP_DMA32 and set __GFP_DMA32 only if the device's dma_mask <
> > > total memory size. By doing this, devices which cannot DMA all the
> > > memory will be limited to ZONE_DMA32, but devices which can DMA all
> the memory will not be affected by this limitation.
> > >
> > > Signed-off-by: Shaohui Xie <Shaohui.Xie at freescale.com>
> > > Signed-off-by: Mingkai Hu <Mingkai.hu at freescale.com>
> > > Signed-off-by: Chen Yuanquan <B41889 at freescale.com>
> > > ---
> > > changes for v2:
> > > 1. use a config option for using two zones (ZONE_DMA32 &
> > > ZONE_NORMAL) in freescale 64 bit kernel.
> > >
> 
> There must have been a misunderstanding. I think this should be a runtime
> choice, possibly by the platform code. Any reason that can't be done ?
> 
[S.H] Do you mean this:

phys_addr_t platform_dma_size (maybe a default value should be used, then platform code will change it)

if (top_of_ram > platform_dma_size)
	max_zone_pfns[ZONE_DMA] = platform_dma_size >> PAGE_SHIFT;
else
	max_zone_pfns[ZONE_DMA] = top_of_ram >> PAGE_SHIFT;

max_zone_pfns[ZONE_NORMAL] = top_of_ram >> PAGE_SHIFT;
	
> Also how does Intel do it ? 
[S.H] below are codes in Intel:

403 void __init zone_sizes_init(void)
404 {       
405         unsigned long max_zone_pfns[MAX_NR_ZONES];
406         
407         memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
408                 
409 #ifdef CONFIG_ZONE_DMA
410         max_zone_pfns[ZONE_DMA]         = MAX_DMA_PFN;
411 #endif
412 #ifdef CONFIG_ZONE_DMA32
413         max_zone_pfns[ZONE_DMA32]       = MAX_DMA32_PFN;
414 #endif
415         max_zone_pfns[ZONE_NORMAL]      = max_low_pfn;
416 #ifdef CONFIG_HIGHMEM
417         max_zone_pfns[ZONE_HIGHMEM]     = max_pfn;
418 #endif
419  
      
For x86_64, there is no CONFIG_HIGHMEM, so there will be three zones: 
ZONE_DMA/ZONE_DMA32/ZONE_NORMAL.

>Do they have iommu and ZONE_DMA32 co-exist ?

I'm not familiar with IOMMU, I read some kernel docs, in Documentation/kernel-parameters.txt,
There are amd_iommu & intel_iommu available, in Documentation/x86/x86_64/boot-options.txt,
It says " Currently four x86-64 PCI-DMA mapping implementations exist". 

Does PPC64 support IOMMU, how to use it?


Best Regards, 
Shaohui Xie