Re: [PATCH 07/19] timberdale: mfd_cell is now implicitly available to drivers

Hi,

On Wed, Apr 06, 2011 at 08:47:34PM +0200, Samuel Ortiz wrote:

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What is a "MFD cell pointer" and why is it needed in struct device?

An MFD cell is an MFD instantiated device.
MFD (Multi Function Device) drivers instantiate platform devices. Those
devices drivers sometimes need a platform data pointer, sometimes an MFD
specific pointer, and sometimes both. Also, some of those drivers have been
implemented as MFD sub drivers, while others know nothing about MFD and just
expect a plain platform_data pointer.

That sounds like a bug in those drivers, why not fix them to properly
pass in the correct pointer?

Because they're drivers for generic IPs, not MFD ones. By forcing them to use
MFD specific structure and APIs, we make it more difficult for platform code
to instantiate them.

I agree. What I do on those cases is to have a simple platform_device
for the core IP driver and use platform_device_id tables to do runtime
checks of the small differences. If one platform X doesn't use a
platform_bus, it uses e.g. PCI, then you make a PCI "bridge" which
allocates a platform_device with the correct name and adds that to the
driver model.

See [1] (for the core driver) and [2] (for a PCI bridge driver) for an
example of what I'm talking about.

The timberdale MFD for example is built with a Xilinx SPI controller, and a
Micrel ks8842 ethernet switch IP. Forcing those devices into being MFD devices
would mean any platform willing to instantiate them would have to use the MFD
APIs. That sounds a bit artificial to me.

do they share any address space ? If they do, then you'd need something
to synchronize, right ? If they don't, then you just add two separate
devices, they don't have to be MFD.

Although there is currently no drivers instantiated by both an MFD driver
and some platform code, Grant complaint about the Xilinx SPI driver moving
from a platform driver to an MFD one makes sense to me. 

I don't think so. That's not really an MFD device is it ? It's just two
different IPs instantianted on the same ASIC/FPGA, right ? Unless they
share the register space, IMHO, there's no need to make them MFD.

[1] http://gitorious.org/usb/usb/blobs/dwc3/drivers/usb/dwc3/core.c
[2] http://gitorious.org/usb/usb/blobs/dwc3/drivers/usb/dwc3/dwc3-haps.c

-- 
balbi