Re: [PATCH/RFC 0/2] simple SPI framework, refresh + ads7864 driver

David Brownell
2005-10-04T20:18:50Z

Hi Vitaly,

> can you please describe the data flow in case of DMA transfer? Thanks!

In the current model, the controller driver handles it (assuming
that it uses DMA not PIO):

- Use dma_map_single() at some point between the master->transfer()
call and the time the DMA address is handed to DMA hardware.

- Probably store those addresses in the spi_transfer struct, using
rx_dma and/or tx_dma as appropriate.

- After the DMA transfer completes, call dma_unmap_single() before
calling spi_message.complete(spi_message.context).

There are two fancier approaches to consider for sometime later, both
of which have been used for several years now by host-side USB.

* SPI controller drivers could require the mappings to already
have been done, and stored in {rx,tx}_dma fields. When those
drivers are using DMA, they'd only use those DMA addresses.

The way host-side USB does that is by having usbcore do work
on all the submit and giveback paths. Currently this SPI
framework doesn't do "core" work on those paths; spi_async()
just calls directly to the controller driver (no overhead!)
and the completion reports likewise go right from controller
driver back to the submitter (also, no overhead).

* Drivers for SPI slave chips might sometimes want to provide their
own rx_dma and/or tx_dma values ... either because they mapped the
buffers themselves -- great for dma_map_sg()! -- or because the
memory came from dma_alloc_coherent().

This implies that the controller drivers are ready to accept
those DMA addresses, and that there are per-buffer flags saying
whether its DMA address is also valid (vs just its CPU address).

Note that the slave-side USB support only supports the latter, which
means the USB peripheral controller drivers with DMA support decide
on a per-request basis whether to do the DMA mappings. I'd lean
towards doing that with SPI; it's a bunch simpler.

Right now DMA isn't on my priority list, but I'd be glad to see
someone else take further steps there. Anyone doing bulk I/O to an
SPI flash chip at 50 MHz is surely going to want DMA for it!

- Dave

p.s. Please be sure to CC me on replies, so I'm sure to see them...