SPI, _sz fields in struct spi_command

[Aleksandr Rybalko]

On Thu, 21 Feb 2013 17:30:03 +0100
Bernd Walter <ticso at cicely7.cicely.de> wrote:

> 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.

Guys, I don't said it will not be supported. :)
I said drivers of controllers who can't will return error in that case,
but other might be ok.

So, conclusion: go-go-go ray! do it please!
:-D


-- 
Aleksandr Rybalko <ray at freebsd.org>