From: Marin Mitov <mitov@issp.bas.bg>
Organization: Institute of Solid State Physics
To: Unai Uribarri <unai.uribarri@optenet.com>
Subject: Re: Workaround hardware bug addressing physical memory
Date: Thu, 15 Jul 2010 19:57:44 +0300
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On Thursday, July 15, 2010 07:46:58 pm Unai Uribarri wrote:
> 
> ----- "Marin Mitov" <mitov@issp.bas.bg> wrote:
> 
> | On 15.7.2010, Unai Uribarri wrote:
> | > Thanks.
> | > ----- "Marin Mitov" <mitov@issp.bas.bg> wrote:
> | > 
> | > | On Wednesday, July 14, 2010 08:06:49 pm you wrote:
> | > | > ----- "Marin Mitov" <mitov@issp.bas.bg> wrote:
> | > | > 
> | > | > | Hi,
> | > | > | 
> | > | > | This is pci driver. You can set dma mask:
> | > | > | 
> | > | > | dma_set_coheren_mask(pdev, DMA_BIT_MASK(31)) 
> | > | > | 
> | > | > | All further alloc_coherent() should be from the region 0-2GB.
> | > | > | 
> | > | > 
> | > | > But I'm using a 64 bit operating system with 32GB of RAM. It's
> | a
> | > | > pity to be unable to use 4GB-32GB range because the 2-4GB range
> | is
> | > | > unusable. So I've written this code to skip invalid areas. Do
> | you
> | > | > think this code could be useful for other drivers?
> | > | 
> | > | Let me summarize if I have correctly understood what you do.
> | > | 
> | > | First, your hardware has problems when the physical (bus) address
> | > | is out of the 0-2GB region, so you cannot use buffers that are out
> | 
> | > | of this range in any case. And the defect is in the peripheral,
> | not in
> | > | the bridge between it and the memory.
> | > 
> | > The hardware works correctly for physical address in the ranges 0 to
> | 2GB
> | > AND 4GB to 32GB. Physical address in the 2-4GB range are read
> | correctly
> | > by the device. But when the device tries to write to them it issues
> | 
> | > invalid PCIe transaction headers: it tries to access such addresses
> | using
> | > a 64-bit transactions when the PCI Express standard mandates to use
> | 32-bit
> | > transactions for memory addresses below 4GB. Some bridges accept
> | such
> | > invalid transactions, but Intel 5500 chipset rejects them.
> | > 
> | > I'm allocating 256MB of RAM for I/O buffers; I'm fear that
> | restricting all
> | > the allocations to the first 2GB of memory will put too much
> | pressure in
> | > that zone of memory. But restricting it to 4GB and above will be
> | okay. 
> | > 
> | > Is there any way to restrict to memory address above 4GB?
> | 
> | Would this work for you?
> | From: Documentation/kernel-parameters.txt:
> | 
> | 	memmap=nn[KMG]$ss[KMG]
> | 			[KNL,ACPI] Mark specific memory as reserved.
> | 			Region of memory to be used, from ss to ss+nn.
> | 			Example: Exclude memory from 0x18690000-0x1869ffff
> | 			         memmap=64K$0x18690000
> | 			         or
> | 			         memmap=0x10000$0x18690000
> 
> I think that linking in a list the unusable memory and freeing it after completing all the allocations is a better solution.

Yes, I agree.

> 
> Thanks.
> 
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