[PATCH v3 1/2] omap3: iovmm: Work around sg_alloc_table size limitation in IOMMU

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Jun 06, 2011 at 06:23:18PM +0200, Laurent Pinchart wrote:
> Hi Russell,
> 
> On Friday 03 June 2011 08:32:12 Russell King - ARM Linux wrote:
> > SG chaining has _nothing_ to do with hardware.  It's all to do with software
> > and hitting the SG table limit.
> 
> What's the reason for limiting the SG table size to one page then ?

As I say, it's got nothing to do with them ending up being passed to
hardware.  Take a look at their definition:

struct scatterlist {
#ifdef CONFIG_DEBUG_SG
        unsigned long   sg_magic;
#endif
        unsigned long   page_link;
        unsigned int    offset;
        unsigned int    length;
        dma_addr_t      dma_address;
#ifdef CONFIG_NEED_SG_DMA_LENGTH
        unsigned int    dma_length;
#endif
};

That clearly isn't hardware specific - hardware won't cope with
CONFIG_DEBUG_SG being enabled or disabled, or whether the architecture
supports the dma_length field, or that this structure has developed from
being:

	void *addr;
	unsigend int length;
	unsigned long dma_address;

to the above over the evolution of the kernel.  Or that we use the bottom
two bits of page_link as our own flag bits?

So no, this struct goes nowhere near hardware of any kind.  It's merely
used as a container to pass a list of scatter-gather locations in memory
internally around within the kernel, especially to dma_map_sg()/
dma_unmap_sg().

If you look at IDE or ATA code, or even SCSI code, you'll find the same
pattern.  They're passed a scatterlist.  They map it for dma using
dma_map_sg().  They then walk the scatterlist and extract the dma
address and length using sg_dma_address() and sg_dma_length() and create
the _hardware_ table from that information - and the hardware table very
much depends on the hardware itself.  Once DMA is complete, they unmap
the DMA region using dma_unmap_sg().

One very good reason that its limited to one page is because allocations
larger than one page are prone to failure.  Would you want your company
server failing to read/write data to its storage just because it couldn't
get a contiguous 8K page for a 5K long scatterlist?  I think if Linux
did that, it wouldn't have a future in the enterprise marketplace.