On Tue, 11 Jun 2013, Alan Stern wrote: > On Tue, 11 Jun 2013, Thomas Pugliese wrote: > > > This patch adds support for scatter gather DMA to the wire adapter and > > updates the HWA to advertise support for SG transfers. This allows the > > block layer to submit transfer requests to the HWA HC without first > > breaking them up into PAGE_SIZE requests. > > While I do not really understand what is going on here, it nevertheless > seems very peculiar. To begin with, whether or not the block layer > breaks up transfers into PAGE_SIZE requests has no connection with SG. > For another thing, the patch has the HCD creating an SG list. That's > not how things work -- the HCD _receives_ the SG list attached to an > URB. > > I'm not sure what problem this patch is intended to solve. Can you > explain in more detail? > > Alan Stern > > Sure. The HWA is a USB connected wireless HCD so the HWA driver communicates with the hardware by sending URBs instead of using mapped memory regions like *HCI devices. There is not necessarily a 1 to 1 correlation between the URBs that come in to the driver and URBs that get forwarded down the stack as HCD commands since there is some encapsulation and segmentation that may be done. Once SG support is enabled in the HWA, the driver now needs to create SGs for the URBs it sends down the stack. In the case of a buffer read URB, it looks like this: 1. The HWA HCD receives a read URB addressed to a device connected wirelessly downstream of the HWA. 2. The HWA driver creates an internal URB to send a Transfer Request command to the HWA on its bulk out endpoint that requests some or all of the data associated with the user's URB. This is analagous to filling in a qTD in EHCI. 3. The HWA driver creates another interal URB to read the Transfer Results back from the HWA's bulk in endpoint. 4. Based on the contents of the Transfer Result. The HWA driver submits a bulk in URB to read the actual data received from the device into the user's buffer. Depending on the amount of data associated with the user's URB, steps 2-4 may be iterated over multiple times due to the limited internal memory of the HWA. My patch updates step 4 to handle URBs that have SGs associated with them. The URB submitted in step 4 will need to include an SG that represents the portion of the origial URB buffer that it is currently reading from the HWA. Tom -- 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
