The biggest problem I have had with the UDC is stalling endpoints from the device. This happens quite a lot when using the mass storage device with Windows as a lot of unsupported (optional) commands are sent.
The problem is that the hardware is (again) trying to be too clever. I set a bit in a register to stall an endpoint. The hardware also has an internal stall bit for each endpoint (not accessible to software). When the host attempts a transaction on an endpoint the hardware checks the internal stall bit and if this is set it sends a stall, if the internal bit is not set it checks the software settable bit and if this is set it latches the internal bit and sends a stall. Resetting the software stall bit does not affect the latched bit, the latched bit can only be cleared by a Clear Feature command from the host. But the Clear Feature is processed by the hardware and the software is not informed. If I leave the software stall bit set there will be another stall on the next transaction so I need to clear the stall bit. I do this by starting a timer to clear the stall bit sometime after I set it. This seems to work most of the time but it's not 100% reliable as I don't know whether the host has actually received the stall (or sent the Clear Feature).
The optimum time delay needed for resetting the stall seems to depend on the speed of the host system, I've tried it with a 2.2GHz P4 and a 300MHz Celeron and it doesn't seem possible to have a value that works well with both systems.
Unless anyone can suggest an alternative that will make this more reliable it seems my only solution will be to implement all the optional mass storage commands that Windows sends so the gadget never has to stall an endpoint. I've already implemented a few of the missing commands and this has improved things. I've also added an option to the mass storage gadget to pad out short replies rather than sending the short reply and stalling (this seems to be a valid alternative according to the mass storage spec) and this also makes it more reliable.
Julian
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