Re: SPI redux ... driver model support

Mark Underwood
2005-09-06T20:10:11Z

--- David Brownell <david-b@pacbell.net> wrote:

> > > > I did think about doing this but the problem
> is how do
> > > > you know bus 2 is the bus you think it is?
> > >
> > > The numbering is board-specific, but in most
> cases
> > > that can be simplified to being SOC-specific.
> ...
> > >
> > > Hotpluggable SPI controllers are not common, but
> > > that's where that sysfs API to define new
> devices
> > > would really hit the spot. ...
> > >
> > > (What I've seen a bit more often is that
> expansion
> > > cards will be wired for SPI, so the thing that's
> > > vaguely hotplug-ish is that once you know what
> > > card's hooked up, you'll know the SPI devices it
> > > has. Then the question is how to tell the
> kernel
> > > about them ... same solution, which again must
> work
> > > without hardware probing.)
> >
> > This is why I decided to pass the cs table as
> platform
> > data when an adapter is registered. This way you
> don't
> > have to try to find out an adapters bus number as
> the
> > adapter has the cs table in it, but because it was
> > passed in as platform data it still abstracts that
> > from the adapter driver. Simple, yet effective :)
>
> Except that it doesn't work in that primary case,
> where the SPI devices
> are physically decoupled from any given SPI (master)
> controller.
> One expansion card uses CS1 for a touchscreen;
> another uses CS3 for
> SPI flash ... the same "cs table" can't handle both
> configurations.
> It's got to be segmented, with devices separated
> from controllers.

With my subsystem that would look like:

static const struct spi_cs_table
platform_spi1_cs_table[] = {
{
.name = "touchscreen",
.cs_no = 1,
.platform_data = NULL,
.flags = SPI_CS_IDLE_HIGH,
.cs_data = 0,
},
{
.name = "flash",
.cs_no = 3,
.platform_data = NULL,
.flags = SPI_CS_IDLE_HIGH,
.cs_data = 0,
},
};

As far as I can see most SPI devices have fixed
wirering to an adapter as SPI is not really a hotplug
bus.
The subsystem does allow you to add extra devices that
aren't in the cs table if you want by calling
spi_device_register in which case you have to setup
the spi_device with the correct information.

>
> Plus, that depends on standardizing platform_data
> between platforms.
> That's really not the model for platform_data! And
> "struct clk" is
> ARM-only for now, too ...

The struct clk should have been removed, I missed it
on that tidy up.
Why not pass platform data through the platform_data
pointer? I have now provided an extra field now which
lets you pass in any other platform data.

>
>
> > Have you looked at the patch which I sent?
> >
>
http://www.ussg.iu.edu/hypermail/linux/kernel/0509.0/0817.html
> >
> > I would appreciate any comments on this approach.
>
> Yes, I plan to follow up to that with comments. As
> with Dmitry's
> proposal, it's modeled closely on I2C, and is in
> consequence larger
> than needed for what it does.

I hope not, I spent a long time learning about the
features of the 2.6 driver model. I'm happy to trim
down any fat.

>
> One reason I posted this driver-model-only patch was
> to highlight how
> minimal an SPI core can be if it reuses the driver
> model core. I'm
> not a fan of much "mid-layer" infrastructure in
> driver stacks.
>

This is what my SPI core tries to do. I would like to
make at 'as small as possible and no smaller'

Mark

> - Dave
>
> -
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