Re: [RFT][PATCH] generic device DMA implementation

[David Brownell <david-b@pacbell.net>]

James Bottomley wrote:
> OK, the attached is a sketch of an implementation of bus_type operations.

Quick reaction ....

Those signatures look more or less right, at a quick glance,
except that allocating N bytes should pass a __GFP_WAIT flag.
(And of course, allocating a mapping needs a failure return.)

That bus_dma_ops is more of a "vtable" approach, and I confess
I'd been thinking of hanging some object that had internal state
as well as method pointers.  (Call it a "whatsit" for the moment.)

That'd make it possible for layered busses like USB and SCSI
to just reference the "whatsit" from the parent bus in their
layered "struct device" objects. [1]

In many cases that'd just end up being a ref to the "platform
whatsit", eliminating a conditional test from the hot path from
your sketch as well as an entire set of new "platform_*()" APIs.

- Dave

[1] That is, resembling what Benjamin Herrenschmidt suggested:

http://marc.theaimsgroup.com/?l=linux-kernel&m=102389432006266&w=2