Re: [spi-devel-general] SPI bus driver synchronous support

[Stephen Street <stephen@streetfiresound.com>]

On Wed, 2006-03-29 at 13:49 -0600, Kumar Gala wrote:
> I was wondering if there was any thought to providing a mechanism for  
> SPI bus drivers to implement a direct synchronous interface so we  
> don't have to use wait_for_completion.
> 
> The case I have is I need to talk to a microcontroller connected over  
> SPI.  I'd like to be able to issue a command to the microcontroller  
> in a MachineCheck handler before the system reboots.  I need a truly  
> synchronous interface opposed to one fronting the async interface.
> 
While is should be possible (it only software after all, right) there
may be complications:

1) The SPI framework relies heavy on the 2.6 driver model and I do not
know much of the system is running in a "MachineCheck handler" before a
reboot.

2) Most of current SPI controller drivers rely on kernel threads to
drive and internal queue. See spi_bitbang.c for an example. Are kernel
threads still running by the time you want to invoke a fully synchronous
transfer? 

3) Some SPI controller drivers are completely interrupt driven with DMA
transfers. See pxa2xx_spi.c for an example.  Are interrupts still
enabled by the time you want to invoke a fully synchronous transfer? 

This in mind it should be possible to add a transfer_sync function
similar to this:

struct spi_master {
	...
	int (*transfer_sync)(struct spi_device *spi,
				struct spi_message *mesg);

	...
};

static inline int
spi_full_sync(struct spi_device *spi, struct spi_message *message)
{
	if (spi->master->transfer_sync) {
		message->spi = spi;
		return spi->master->transfer_sync(spi, message);
	}

	return -EOPNOTSUPP;
}

Off course, all SPI controller drivers would need some kind of interlock
between the standard transfer and transfer_sync.

Do you have SPI hardware or are you using spi_bitbang?  If you have
dedicated hardware what type?

What do you think David?

> Also, who is the maintainer for the SPI subsystem?
> 
David Brownell

-Stephen