Re: highmem, aic7xxx, and vfat: too few segs for dma mapping

[Gérard Roudier]

On Wed, 12 Dec 2001, David S. Miller wrote:

>    From: Gérard Roudier <[e-mail]>
>    Date: Wed, 12 Dec 2001 21:24:59 +0100 (CET)
>
>    A N% loss for the 99% case in order to support the 1% is close to N%
>    loss. So, each time we bloat or complexify the code with no relevance for
>    the average case, the overall difference cannot be a win.
>
> Do you know, you can use this N% loss to implement handling of the
> very problem you have wrt. sym53c8xx hw bugs? :-)

What hw bugs ? :-)

The driver is very fortunate. Based on DELs, only a few known hw bugs have
had to be worked-around by sym drivers. In fact, you can avoid most bugs,
by taking into account usual bugs in PCI hw. When I have made sym53c8xx
from ncr53c8xx, I had some clues in mind.

The 16MB/32MB boundary I pointed out can only affect sym53c896 rev. 1, and
only in not very likely situations. In plan to resurrect the work-around
in Linux sym_glue.c if pci_map_sg() will not want to handle the issue.

So, on paper, you were talking about a single unlikely to happen hw bug.
:-)

> To be honest all the machinery to handle the problems you have
> described are there today, even with IOMMU's present.  The generic
> block layer today knows when IOMMU is being used, it knows what kind
> of coalescing can and will be done by the IOMMU support code (via
> DMA_CHUNK_SIZE), and therefore it is capable of adhering to any
> restrictions you care to describe to the block layer.

Will look into all these great things over the week-end and let you all
know my remarks if any.

> It's only a matter of coding on Jens's part :-)

Jen's seems to be a great and very courageous coder, so let me trust him
not to miss anything important. :)

Thanks for your reply,
  Gérard.

PS: Don't take the wrong way my statements against Sun stuff. In fact, I
    dislike almost everything that comes and came from them. :)