Re: RFC: Platform data for onboard USB assets

From: Andy Green
Date: Fri Mar 18 2011 - 13:52:27 EST

On 03/18/2011 03:00 PM, Somebody in the thread at some point said:

Hi -

You changed your first opinion about tagging "dynamically probed
devices" with what is effectively platform_data, cool.

I still don't like the idea of attaching platform_data to more
devices, when we try to move people away from that in other
parts of the kernel, because of the known deficiencies.

Whatever way you look at it, data delivered into the driver by Device Tree is fundamentally the same action as delivering data into the driver by platform_data. Yes, you query by named element with device context, but you end up with the same answer as if you dereference a platform_data member. There are no "known deficiencies" to platform_data for this action either, at least, not known to me, I don't think lack of typechecking on the pointer itself is an issue given the accuracy it can be targeted to a soldered-on-the-board device.

Passing a MAC address in a device tree property is a
well-established method that is used on many drivers, and
is portable across operating systems and architectures.

If you're talking about Device Tree, that itself is not at all "well established" let alone servicing drivers from it. Like I say I don't want to seem like I am down on it, but it is very new indeed let's face it and few drivers are using it for functional configuration information compared to vast numbers using platform_data.

=====> If Device Tree APIs is mandated to implement functionality fixes to drivers and platform_data is blocked for this, then we end up with different, rotting functionality for platform_data basis and drivers that remain broken on the many, many, platforms that don't have and will never have Device Tree. That does NOT sound like the right approach.

I guess the grand plan is to eliminate platform_data by overwhelming it with Device Tree refactoring. But each driver has to be tested and each board definition file changed... that is a huge, huge undertaking that will not happen in any kind of medium and perhaps not long term either.

So they will have to coexist for a very long while. A policy of denying fixes to platform_data users by enforcing introduction of Device Tree APIs and making platform_data users out as troglodytes does not sound workable.

We don't need to implement the entire binding. My point was that
if we implement a way to attach a device_node to a usb_device, we
should do it in a way that is compatible with that binding, rather
than coming up with a new way.

That document is of no interest outside open firmware / Device Tree implementation since it is specific to it, and there is no value to reference it for a platform_data based solution.

My impression so far is that attaching a struct device_node to
static USB devices can be useful in general, but I wouldn't
go so far to suggest using this for dynamically probed devices.

At least we agree there's no point to target pluggable devices with either solution, in which case platform_data and Device Tree provide the same end result, plus or minus extra query API.

By the way I intend shortly to extend my patchset to cover Panda WLAN case via probed MMC / SDIO device in the same way as USB. So there will then be a second use case for my async platform_data patchset via a different subsystem. Of course, maybe it just doubles the number of beatings ^^

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