Subject: Re: Rev 1.19 of busdma.doc
To: Justin T. Gibbs <>
From: Jason Thorpe <>
List: tech-kern
Date: 11/11/1996 21:59:02
On Sat, 09 Nov 1996 17:26:59 -0800 
 "Justin T. Gibbs" <> wrote:

 > There is nothing here preventing it, but it would be nice to explicitly
 > allow the system to choose an inline variant of the specified function if it
 > is 'known' to the system.  For example, I expect many of the SCSI drivers
 > to use a single common routine to do this, and I don't see any reason why
 > some arches might not want to short circuit the function calls.  I think
 > you should also need to pass the index of the SG entry you are creating
 > for reasons I'll make clear below.

As I've stated before, the fact that several PC SCSI controllers may
use a similar (or identical) s/g list format is simply irrelevant.  We
should pick an abstraction and stick with it.

 > How can you make this guarantee if you are going to support "compaction" of
 > the address to fit the limits of the target SG list.  I could imagine the
 > implementation attempting the map first and then resorting to compaction
 > and starting over with the first segment, only after it has exausted all of
 > the SG segments.  The only way to know if you fit is to walk the pages and
 > while you are walking the pages, you might as well fill in th SG values as
 > you go.  This is why I think this guarantee should be replaced by an
 > explaination as to why it may be called more than "nsegments" times and the
 > index should be added to bus_dmamap_load_func_t.

I'm not sure what you mean by "compaction" here... For the purpose of
my answer, I'll assume that you mean "single addr/len pair for multiple
adjacent pages".

Passing an index to the callback is not only unnecessary, but it may
not make sense in the context of the callback function.  For example,
what if the s/g descriptor for a device is implemented as some sort
of FIFO?

The correct way for this to happen is for bus_dmamap_load() to "look ahead"
and perform compaction on-the-fly, conforming to the constraints of
the device as specified in bus_dmamap_create().  By doing this, you
eliminate unnecessary calls to the callback altogether.  This is really
just a trivial implementation detail, and doesn't warrant adding
complexity to the interface.

I'm sticking with the definition of the callback being called at most
"nsegments" times, and not adding an index argument to the callback.

 > We also need an additional "BUS_DMA_COMPACTTOFIT" flag so that clients have
 > to explicitly ask for compaction to occur.  This allows clients that can
 > easily break up transfers to split the data up by filling their SG list up
 > and passing the residual back up to caller that generated the transaction.
 > At the same time, it cleanly addresses the needs of things like the st
 > driver.  For this to completly work, bus_dmamap_load would have to somehow
 > report the residual on error.

Uhh... no.  First of all, the "st" driver shouldn't be at all concerned
with how data is transfered at the lower level.  In fact, the "st" driver
has no way to pass such a flag to bus_dmamap_load().  I don't see how
this cleanly addresses anything.

Secondly, in my little world, these DMA transactions are "atomic".  It's
all or nothing.  I don't want to supply the rope that would allow for
"most" of a transaction to occur, and have the transfer of the residual
fail.  That stands the possiblity of introducing a lot of extra code

Jason R. Thorpe                             
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