Re: USB Transceiver & USB Gadget driver doubts

[David Brownell <david-b@xxxxxxxxxxx>]

On Thursday 20 December 2007, Midhun A wrote:
> Hi All,
> 
>     I have started to write the gadget driver for i.MX21 (CSB535FS
> Board). I am writing it based on the OMAP driver present in the
> mainline kernel.

Are those controllers at all similar?

And I'm quite sure I've seen an i.MX peripheral controller driver
posted somewhere ... maybe a buggy one, and certainly it's not
been merged to mainline, but it may well be easier to start with
that one than write a new one.


> I have the following doubts: 
> 
> 1. I configured the OTG of i.MX21 as a function only (and not as OTG).
> On the hardware side, I dont have an OTG transceiver (I think it comes
> with I2C control) but rather the normal USB tranceiver (Fairchild -
> USB1T11A). Is this fine?

It's certainly a useful configuration.  Just be aware that when
someone makes a fully OTG-capable version, things may need to
be shuffled around ... and make that shuffling easier.


> 2. In the driver, I have just implemented the probe of UDC as of now.
> I see a method for pullup disable in the OMAP driver. What is this
> for? Where is this pull up used?

It's used to control enumeration.  When the pullup is disabled,
the host won't detect a peripheral on that line.  When it's
enabled, it will detect the peripheral.  It's under software
control, since the peripheral should be invisible until it's
able to respond to the host.


> Is this specific for OMAP? 

Every USB peripheral controller should offer the option to
manage the signal pullup in software.  See the USB 2.0 spec;
it's a D+ pullup on everything except low speed devices,
where it's a D- pullup.  In the absense of SRP, hosts detect
peripherals by seeing one of the two signal lines pulled up

So while the specific implementation of that pullup may be
specific to OMAP, 

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