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Date: Fri, 09 Mar 2001 11:42:38 -0800
From: David Brownell <david-b@pacbell.net>
Subject: Re: [linux-usb-devel] Re: SLAB vs. pci_alloc_xxx in usb-uhci patch
 [RFC: API]
To: Johannes Erdfelt <johannes@erdfelt.com>,
        linux-usb-devel@lists.sourceforge.net
Cc: Manfred Spraul <manfred@colorfullife.com>,
        Russell King <rmk@arm.linux.org.uk>, zaitcev@redhat.com,
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> >  > Do lots of drivers need the reverse mapping? It wasn't on my todo list
> >  > yet.
> > 
> > I am against any API which provides this.  It can be extremely
> > expensive to do this on some architectures, 

The implementation I posted needed no architecture-specific
knowledge.  If cost is the issue, fine -- this makes it finite,
(not infinite), and some drivers can eliminate that cost.


> >        and since the rest
> > of the PCI dma API does not provide such an interface neither
> > should the pool routines.
> 
> The API I hacked together for uhci.c didn't have this.

But it didn't handle the OHCI done-list processing, and we've heard
a lot more about pci_*_consistent being needed with OHCI than
with UHCI; it's more common on non-Intel architectures.

Given that some hardware must return the dma addresses, why
should it be a good thing to have an API that doesn't expose
the notion of a reverse mapping?  At this level -- not the lower
level code touching hardware PTEs.

- Dave


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