We are working on a Host Controller Driver for the onboard host controller of the Renesas SH7760 chip (SH4 CPU). We are using the 2.4.21 kernel.
This controller is OHCI compatible but:
1. It is not PCI
2. It uses an 8Kb on-chip shared memory area rather than main memory for descriptors and buffers
We are basing our driver on an earlier driver for the SH7727 which is also not PCI but does use main memory for buffers, this driver was itself based on the standard OHCI driver (derived from kernel 2.4.18).
The problem is how to (easily) deal with the onchip memory. We will have to copy USB data between the main memory and the onchip buffer and there seem to be a lot of places where this is necessary.
Why? You can allocate EDs and TDs (and HCCA) there in the first place, so the only copying would be for data buffers (where usb-ohci or ohci-hcd have pci dma mapping calls).
Our other worry is the size of the shared memory area. Is only having 8Kb likely to cause problems?
You'll have to write a memory manager for that. I suspect it'd behave reasonably well with 4KB for EDs/TDs/HCCA, and the rest for data buffers. That's at least three frames' worth of data.
- dave
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