SPI, _sz fields in struct spi_command

[Bernd Walter]

On Thu, Feb 21, 2013 at 10:21:00AM -0500, Patrick Kelsey wrote:
> >On Thu, Feb 21, 2013 at 9:30 AM, Aleksandr Rybalko <ray at freebsd.org> wrote:
> >> On Thu, 21 Feb 2013 02:44:33 +0100
> >> Bernd Walter <ticso at cicely7.cicely.de> wrote:
> >>
> >> On Thu, Feb 21, 2013 at 02:26:55AM +0200, Aleksandr Rybalko wrote:
> >> > 2. teach consumers to give only correct numbers (very nice we have only
> >> > two SPI devices in tree)
> >> >
> >> > After that we will be able to make drivers for some (potential) devices
> >> > which will require bidirectional communication. And controllers which
> >> > can't do that, will just report error in that. I believe peoples thinks
> >> > before attach such devices to controllers, so we will not have such
> >> > incompatibility.
> >>
> >> I don't think there are many devices requiring RX/TX at the same time.
> >
> > Anyway, we will be able to do that, and we don't care now because don't
> > have such drivers yet.
> >
> 
> Taking the view that "RX/TX at the same time" means that one wants to
> send meaningful data to the slave device at the same time one is
> interested in what data is returned during that transmission, there
> are such devices in use out there.  Linear Technologies has several
> ADCs, such as the LTC2446, for which you obtain the previous
> conversion result while sending the configuration bits for the next
> conversion to be performed.

Forgot about ADC with channel selection.

> Although this is slightly out of focus for the specific issue
> originally raised, while on the topic of things that need to get done
> on SPI in real systems, there are also devices out there that require
> specific data or some number of clocks to be provided while chip
> select is deasserted.  One example of the former is the LTC2404, which
> is a multichannel ADC for which the input channel for the next
> conversion is selected by the last four bits clocked in *before* chip
> select is asserted.  One example of the latter is the spec for SPI
> access to MMC/SD cards, which requires a certain number of clocks to
> be applied with chip select deasserted in order to initialize the
> card.  If you ever find yourself wondering why an SPI software
> interface provides independent bus acquisition and chip select
> control, the reason is to support these types of devices.

With many ADC you also want probing support.
Assign CS and GPIO-read MISO for ready without clocking.
Some flash chips also work this way.
Not sure if AT45DB support this and how our driver works.
With own projects I usually ask AT45DB about ready state by transfering
a status word.

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.