Date: Mon, 23 Oct 2000 16:07:03 -0700 From: Mike Smith <msmith@freebsd.org> To: "Chris Ptacek" <ptacek@technologist.com> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: DMA in drivers? Message-ID: <200010232307.e9NN73h05149@mass.osd.bsdi.com> In-Reply-To: Your message of "Sun, 22 Oct 2000 21:26:11 PDT." <NEBBKLFHMJGHPAEKCHJJEEHNCCAA.ptacek@technologist.com>
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> I am in the process of writing a PCI driver for an encryption card. The
> specifications state that the DMA Destination Address, DMA Dest. Length, DMA
> Source Addr, and DMA Source Length should be loaded into registers in the
> card. Part of the info states:
>
> "This register is used to establish the PCI address for data moving from the
> the Host Computer Memory to the card. It consists of a 30 bit counter with
> the low-order 2 bits hardwired as zeros. The address stored may be any
> nonzero byte length that is a multiple of 8, since 8 bytes are required to
> make up a DES encryption block. The Source Address Register is continually
> updated during the transfer process and will always be pointing to the next
> unwritten location."
>
> What do I need to do to get a memory address for the source and destination
> data for the DMA transfers? It sounds as if these memory addresses must have
> the last three bits zeros, will this happen automatically? Right now I am
> stuck on how this DMA stuff is working and any help would be appreciated.
> Oh yeah, I am targeting this driver for a FreeBSD 3.x system.
3.x is obsolete, and should typically not be considered for new drivers
unless you're doing something specifically to spec.
Having said that, the busdma infrastructure in 3.x probably works well
enough to do what you want.
The actual process to follow is going to depend largely on how your
exported interface to the driver works. The card sounds like it's pretty
stupid (eg. no scatter/gather DMA, no descriptor ring, etc). so you
really have two options:
- manual scatter/gather
- copy to/from a conforming DMA-able buffer.
The choice between these two depends on the nature of the data; if your
chunks of contiguous data are relatively large, then manual scatter/
gather will be more efficient. OTOH, if the chunks are small, then
it will be more efficient to gather the stream into a single buffer and
pass that to the adapter instead.
In either case, you will want to look to the busdma code. At the very
least, you will need to construct a DMA tag which decsribes the card's
DMA capabilities and a DMA map for the transaction in progress.
For manual scatter/gather, you then pass the input segment to
bus_dmamap_load and use the helper function to build a list of physical
segment addresses and lengths. Then you feed each of these segments to
the adapter, and call bus_dmamap_unload when you're done.
To allocate a conforming DMA-able buffer, use bus_dmamem_alloc and then
pass it to bus_dmamap_load in order to obtain the base address of the
buffer (note that if you've set the tag up properly, the buffer will be
physically contiguous).
--
... every activity meets with opposition, everyone who acts has his
rivals and unfortunately opponents also. But not because people want
to be opponents, rather because the tasks and relationships force
people to take different points of view. [Dr. Fritz Todt]
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