linux-kernel - Re: Workaround hardware bug addressing physical memory

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Date:	Thu, 15 Jul 2010 10:08:26 +0200 (CEST)
From:	Unai Uribarri <unai.uribarri@...enet.com>
To:	Marin Mitov <mitov@...p.bas.bg>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Workaround hardware bug addressing physical memory

Thanks.
----- "Marin Mitov" <mitov@...p.bas.bg> wrote:

| On Wednesday, July 14, 2010 08:06:49 pm you wrote:
| > ----- "Marin Mitov" <mitov@...p.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?

| Second, you do allocations and check if dma_addr_t is in this range.
| If it is, you keep it for use. If not, you put it in a list and
| allocate a new one.
| When you allocate sufficiently memory in 0-2GB region, you deallocate
| 
| the memory (kept in the list) that is out of the region. Right?
| 

Yes, except that the validator function is:

static int validate_dma(void *ptr, struct device *dev, size_t size, dma_addr_t handle)
{
        return handle < 0x80000000U || handle > 0xFFFFFFFFU;
}

Thanks.
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