Mike Frysinger wrote:
On Wednesday 06 February 2008, Ben Warren wrote:
Haavard Skinnemoen wrote:
On Tue, 5 Feb 2008 23:34:46 -0500 Mike Frysinger vapier@gentoo.org wrote:
there isnt a function to pair up with spi_setup() ? for example, the normal communication flow with a SPI flash:
- spi_setup - turn on SPI
- spi_cs_activate - assert CS
- spi_xfer -
- op code (read/write/erase)
- address
- actual block data
- spi_cs_deactivate - deassert CS
- ??? - turn off SPI
Right. I thought of spi_setup() more as a function that needs to be called one time per slave to set up communications parameters, not really for turning the SPI on as such.
But perhaps it would make sense to combine those two functions. How about we turn it into
/* Set slave-specific parameters and enable SPI */ int spi_claim_bus(int cs, unsigned int max_hz, unsigned int mode);
/* Disable SPI */ void spi_release_bus(int cs);
The claim/release naming also makes it clear that the SPI device driver has exclusive access to the bus between those two calls.
If there really is a need to turn off the controller, or change the transfer rate on the fly, then this is good. OTOH, this is a bootloader, not an OS, and probably the vast majority of use cases would just be to initialize the controller to a speed that all devices can handle, transfer some data to/from one or more devices, then boot an OS. Maybe some people need to do more, I don't know.
U-Boot's design principles dictates that you get in, do your thing, and get out. getting out means breaking down/releasing/turning off/however you want to describe it. there is also the possibility of slight power savings as Haavard points out. you could also have board functions that reuse the pins for some other purpose (say they have muxing in place or something).
True. I just want to be careful we don't over-engineer this...
also, what's the deal with spi_xfer() taking a length in # of bits ? is it realistic to transmit anything less tan 8 bits ? the Linux kernel driver does not support this, so it cant be a big need ...
I don't know. That's unchanged from the original API. But I certainly wouldn't object if we turned it into a length in bytes.
I seem to remember working with a Broadcom device where some of the transfers were odd numbers of nibbles (e.g. 12 bits). Not necessarily a reason to keep bit granularity, but I don't see a reason to artificially limit things either.
but is there any real spi controllers that can transmit less than a byte at a time ? i guess if you consider gpio-based soft spi ... -mike
Sure, the Freescale SPI controller that I wrote a driver for (MPC8xxx) can send an arbitrary number of bits. Not sure exactly where that's useful, but my worldview is limited to high-powered telecom/datacom equipment.
regards, Ben