[PATCH 10/14] tegra: usb: Add support for USB peripheral

[Stephen Warren]

On 12/06/2011 02:23 PM, Simon Glass wrote:
> Hi Stephen,
> 
> On Tue, Dec 6, 2011 at 12:42 PM, Stephen Warren <swarren at nvidia.com> wrote:
>> On 12/05/2011 06:14 PM, Simon Glass wrote:
>>> Hi Stephen,
>>>
>>> On Mon, Dec 5, 2011 at 4:17 PM, Stephen Warren <swarren at nvidia.com> wrote:
>>>> On 12/05/2011 04:35 PM, Simon Glass wrote:
...
>>>>> Also the only way I can see it being hard-coded is by the kernel
>>>>> looking at the peripheral address and converting this to an ID. That
>>>>> seems really ugly to me. Or am I missing something?
>>>>
>>>> Well, the Tegra SD/MMC driver works exactly that way (well, mapping an
>>>> instance ID to both base address and periph ID), so there's certainly
>>>> precedent for this. And that driver is not unique.
>>>
>>> I don't know if I can NAK a comment but I would like to NAK this one.
>>
>> I don't know what that means; that you believe my statement is
>> incorrect, or you don't like the argument I'm basing on it or ...?
> 
> What happens is that the SD/MMC driver (dating from pre-FDT days) has
> a hard-coded base address and peripheral ID, based on an instance ID
> (0, 1, 2).

That's basically the exact same thing, it's just the exact fields that
are the key into and output from the table are different.

> I would expect that we want the FDT to control all of this - it
> already has the base address, can have the peripheral ID, and the
> instance ID (ordering) should be set by the FDT not hard-coded IMO.
> That's the reason for my NAK comment. It just seems completely wrong
> to duplicate this information in the driver and require the instance
> ordering to be hard-coded in the driver. It means that boards that
> want to change this ordering must fiddle around in the driver to make
> this work.
> 
> Also this is U-Boot, not the kernel. Commands like 'ext2load' require
> that you pass the instance ID to indicate which device to use.

SD/MMC is a very different use-case, and not a good analogy to USB.

With SD, the user always wants to identify a specific device that they
know contains their files.

With USB, the user doesn't care that there are multiple USB host
controllers in the SoC; they just want to plug in their device somewhere
and have it work. Having to decide which USB controller to enable at a
time is pretty hokey, given there's unlikely to be any indication at all
on the PCB or case which ports map to which USB controllers.

What the user might care about is selecting a enumerated particular USB
device. They'd only know which one after looking at some "lsusb" command
(or whatever it's called in U-Boot) in the general case.

Hence, I assert that USB host controller number is completely
irrelevant, and all should be active at once.

This of course is all somewhat moot given that I pointed out
ehci-tegra.c only supports one of the hosts anyway...

> OK, so is your objection that we ignore a peripheral that has no
> alias?

Yes.

And that enumeration is based on alias nodes not enumerating all nodes
that match the compatible flag.

> If I change the code to locate these and add them at the end
> would that be better?

Sure.

I think the best implementation would be to enumerate everything solely
based on compatible flags, and allowing "usb start" to accept either an
alias name (which would be fixed) or a DT unit address or node name
(which would be fixed) or a controller index (which might vary based on
enumeration order) to select the controller.

The next best implementation would be to enumerate solely based on
compatible flags, then sort the list based on alias, thus allowing alias
to cause a static order.

However, enumerating based on alias, then enumerating based on
compatible flags and adding any missing nodes would be fine.

>> As I mentioned elsewhere, the patches only allow one to actually use usb
>> controller 0; ehci-tegra.c's ehci_hcd_init() hard-codes port 0 when
>> calling tegrausb_start_port(). Rather than relying on non-standard
>> aliases usage to restrict the set of USB devices that get instantiated,
>> why not just add status = "disabled" to all but one USB host's node in
>> each board's .dts file; that's the standard way to disable devices.
> 
> I can do that, but how do I solve the ordering problem?

If you only have one enabled controller in the .dts file, there can't be
any ordering issue since there is always only 1 controller.

-- 
nvpublic