Re: [LIP] pci_alloc_consistent usage in x86-64 kernel

Hi Arun,
Thanks for your reply.
My device is 64-bit DMA capable and I am passing 64bit DMA as mask.
but as per the following code in pci-gart.c in arch/x86-64/kernel of 2.4.20


if (hwdev == NULL || hwdev->dma_mask < 0xffffffff || no_iommu)
		gfp |= GFP_DMA;
If there is no_iommu then without considering the dma_mask the GFP_DMA is 
set and the memory allocation is limited.

then as per the following code. for all the virtual memory allocated through 
pci_alloc_consistent, If the address is beyond 4GB then the iommu is used 
without considering the dma_mask.
                     {
		int high = 0, mmu;
		if (((unsigned long)virt_to_bus(memory) + size) > 0xffffffffUL)
			high = 1;
		mmu = high;
		if (force_mmu && !(gfp & GFP_DMA))
			mmu = 1;
		if (no_iommu) {
			if (high) goto error;
			mmu = 0;
		}
		memset(memory, 0, size);
		if (!mmu) {
			*dma_handle = virt_to_bus(memory);
			return memory;
		}

But I have got another finding that with Few makes of motherboard with same 
kernel I am able to allocate more than 4GB. I am confused about the 
behaviour of the above code which I predicted earlier.

As I dont have hardware resources here I couldnt debug much.

If you can give me some pointers it will be of great help.

Thanks
Sathya







>From: Arun Sharma <[email protected]>
>Reply-To: linux-india-programmers-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org
>To: linux-india-programmers-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org
>Subject: Re: [LIP] pci_alloc_consistent usage in x86-64 kernel
>Date: Sun, 30 May 2004 10:24:30 -0700
>
>On Thu, May 27, 2004 at 08:09:14PM +0000, Sathya Prakash V wrote:
> >
> > By debugging I am able to identify the iommu_alloc function in 
>pci_gart.c
> > is returning fail.
>
>iommu is needed only for those PCI devices which are not able to DMA
>beyond 32 bits i.e. 4GB. In those cases, some platforms implement a
>piece of hardware that sets up a 32 bit bus address for the PCI device
>to see, but which is mapped to a 64 bit physical address i.e. any place
>in physical memory. x86-64 hacks the AGP GART hardware to implement
>this functionality.
>
>Is your emulated device capable of 64 bit DMA ? If yes, why is it
>passing a dma_mask of less than 4GB ?
>
>	-Arun
>
>
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