Hi all, While working on the i.MX21 HCD I have a few questions on how hardware resource limits should be handled. The hardware has 32 "ETDs", my driver allocates an ETD for each active endpoint. That is the ETD is allocated in the urb_enqueue() function IF there is no URB already equeued for the endpoint and is freed when there are no more URBs for the endpoint. If no more ETDs are available can the driver return a failure status for urb_enqueue() or must it queue locally? In the former case which status (ENOMEM, ENXIO ??) In the latter should there be a limit on the queue size? As isoc transfers use 2 ETDs for double buffering this limits the number of non-isoc active endpoints to 32 - 2*isoc Furthermore the hardware has 4K of "data memory" which must be allocated to transfers. Non isoc transfers each require 2*maxpacket of data memory. With no isoc transfers worst case data memory usage (full speed only) 32 * 64 * 2 = 4K => no data memory exhaustion. When isoc transfers come into play its gets a bit more complicated. Taking table 5-4 from the USB spec and eliminating the cases that are impossible for this hardware (32 ETD limit, 2 ETDs/isoc => 16 max simultaneous). Then calculating memory usage: Payload #Transfers DMEM used ETDs used DMEM left ETDs left DMEM/ETD maxPacket max64 128 10 2560 20 1536 12 128 64 12 256 5 2560 10 1536 22 69 34 12 512 2 2048 4 2048 28 73 36 16 1023 1 2046 2 2050 30 68 34 16 maxPacket = max packet size for non isoc endpoints if all available ETDs used max64 = number of 64 byte endpoints that could be supported concurrently with the isoc transfers So this shows the 128byte payload cannot not exhaust memory either but the others could. In fact it could be worse because the 4K data memory is shared between the USB host and USB function modules although there is no gadget driver code yet. My current idea is : 1) urb_enqueue() will fail for all transfer types if no ETD is available. 2) urb_enqueue() for isoc transfers will attempt to allocate data memory immediately (and fail if there isn't enough). 3) urb_enqueue() for non isoc transfers will not allocate data memory. 4) when data memory for non isoc transfers is required and not available the transfer will be put on a "pending" queue and processed when memory becomes available. Is this reasonable? Thanks, Martin -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
