On Thu, 6 Dec 2012, Andy Green wrote:
> > Right. The question is should it be done in this somewhat roundabout
> > way (you've got two separate assets for setting up this one thing), or
> > should the USB port code be responsible for explicitly checking if
> > there are any applicable assets when the port is created?
> >
> > The advantange of the second approach is that it doesn't involve
> > checking all the ancestors every time a new device is added (and it
> > involves only one asset). The disadvantage is that it works only for
> > USB ports.
>
> It's done in two steps to strongly filter candidate child devices
> against being children of a known platform device. If you go around
> that, you will be back to full device path matching with wildcards at
> the USB child to determine if he is "the one". So that's a feature not
> an issue I think.
I don't follow. Suppose an asset is attached to ehci-omap.0 with its
string set to "-0:1.0/port1" and the other members pointing to the
regulator, clock and so on. And suppose the USB port code, when
creating a new port, checks for a name match against all the assets
attached to its bus's host controller device structure. That should
do exactly what you want while using only one asset.
> I can see doing it generically or not is equally a political issue as a
> technical one, but I think if we bother to add this kind of support we
> should prefer to do it generally. It's going to be about the same
> amount of code.
Not quite. If you do it generally then you have to insert hooks at all
the places where a subsystem _might_ need it. If you do it
specifically then no hooks are needed at all -- just a match call at
the right place in the subsystem that needs it.
> As Tony Lindgren said, even board-omap4panda.c itself is deprecated, to
> project platform info into USB stack you either have to add DT code into
> usb stack then to go find things directly, or provide a generic
> methodology like assets which have the dt bindings done outside of any
> subsystem and apply their operations outside the subsystem too.
Assuming DT can be extended to support assets, adding one asset will be
simpler than adding two.
> > To answer the question, we need to know how other subsystems might
> > want to use the asset-matching approach. My guess is that only a small
> > number of device types would care about assets (in the same way that
> > assets matter to USB ports but not to other USB device types). And it
> > might not be necessary to check against every ancestor; we might know
> > beforehand where to check (just as the USB port would know to look for
> > an asset attached to the host controller device).
>
> Yes I think it boils down to "buses" in general can benefit from this.
> They're the thing that dynamically - later - create child devices you
> might need to target with what was ultimately platform information.
>
> On Panda the other bus thing that can benefit is the WLAN module on
> SDIO. In fact that's a very illuminating case for your question above.
> Faced with exactly the same problem, that they needed to project
> platform info on to SDIO-probed device instance to tell it what clock
> rate it had been given, they invented an api which you can see today in
> board-omap4panda.c and other boards there, wl12xx_set_platform_data().
> This is from board-4430sdp:
>
> static struct wl12xx_platform_data omap4_sdp4430_wlan_data __initdata = {
> .board_ref_clock = WL12XX_REFCLOCK_26,
> .board_tcxo_clock = WL12XX_TCXOCLOCK_26,
> };
>
> ...
>
> ret = wl12xx_set_platform_data(&omap4_sdp4430_wlan_data);
>
> You can find the other end of it here in
> drivers/net/wireless/ti/wlcore/wl12xx_platform_data.c
>
> static struct wl12xx_platform_data *platform_data;
>
> int __init wl12xx_set_platform_data(const struct wl12xx_platform_data *data)
> {
> if (platform_data)
> return -EBUSY;
> if (!data)
> return -EINVAL;
>
> platform_data = kmemdup(data, sizeof(*data), GFP_KERNEL);
> if (!platform_data)
> return -ENOMEM;
>
> return 0;
> }
>
> when the driver for the device instance wants to "get" its platform data
> it reads the static pointer via another api. Obviously if you want two
> modules on your platform that's not so good.
Also it isn't type-safe, although that's probably not a big concern.
> I doubt that's the only SDIO device that wants to know platform info.
> So I think by providing a generic way we help other buses and devices.
I agree, assets look like a good way to improve this. In fact, to some
extent assets appear to be a generalization or extension of platform
data.
Alan Stern
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