Re: [PATCH] usb: wire adapter: add scatter gather support

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On Tue, 11 Jun 2013, Thomas Pugliese wrote:

> 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.

I see.  Thanks for the explanation.  Things do get confusing when you 
have a USB-connected host controller!

Alan Stern

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