Re: libata .sg_tablesize: why always dividing by 2 ?

[Mark Lord <liml@rtr.ca>]

Benjamin Herrenschmidt wrote:
> On Mon, 2008-02-25 at 19:15 -0500, Jeff Garzik wrote:
>> Mark Lord wrote:
>>> Jeff,
>>>
>>> We had a discussion here today about IOMMUs,
>>> and they *never* split sg list entries -- they only ever *merge*.
>>>
>>> And this happens only after the block layer has
>>> already done merging while respecting q->seg_boundary_mask.
>>>
>>> So worst case, the IOMMU may merge everything, and then in
>>> libata we unmerge them again.  But the end result can never
>>> exceed the max_sg_entries limit enforced by the block layer.
>> <shrug>  Early experience said otherwise.  The split in foo_fill_sg() 
>> and resulting sg_tablesize reduction were both needed to successfully 
>> transfer data, when Ben H originally did the work.
>>
>> If Ben H and everyone on the arch side agrees with the above analysis, I 
>> would be quite happy to remove all those "/ 2".
> 
> The split wasn't done by the iommu. The split was done by the IDE code
> itself to handle the stupid 64k crossing thingy. If it's done
> differently now, it might be possible to remove it, I haven't looked.
..

The block layer uses seg_boundary_mask to ensure that we never have
to split them again in the LLD.

A very long time ago, when I wrote the IDE DMA code, this was not the case.

Cheers