Re: [RFC] usb: xhci: add Immediate Data Transfers support

[Nicolas Saenz Julienne]

Hi Felipe, thanks for the review :)

On Fri, 2019-02-01 at 15:31 +0200, Felipe Balbi wrote:
> Hi,
> 
> Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx> writes:
> > Immediate data transfers (IDT) allow the HCD to copy small chunks of
> > data (up to 8bits) directly into its output transfer TRBs. This avoids
>               ^^^^^
>               8 bytes

Obviously, noted.

> 
> > the somewhat expensive DMA mappings that are performed by default on
> > most URBs submissions.
> > 
> > In the case an URB was suitable for IDT. The data is directly copied
> > into the "Data Buffer Pointer" region of the TRB and the IDT flag is
> > set. Instead of triggering memory accesses the HC will use the data
> > directly.
> > 
> > An additional URB flag was created to mark whenever the URB doesn't need
> > any DMA mapping. Ideally it would have been nice to use a private flag
> > as this is specific to XHCI. Yet it's not possible as the URB private
> > area is allocated only after the DMA mapping is done.
> > 
> > Isochronous transfers are not implemented so far as it's hard to find a
> > device that will send such small packets.
> > 
> > This was tested using USB/serial adapter and by controlling the leds on
> > an XBOX controller. There where no disruptions on the rest of USB
> > devices attached on the system.
> > 
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
> > ---
> >  drivers/usb/host/xhci-ring.c |  6 ++++++
> >  drivers/usb/host/xhci.c      | 37 ++++++++++++++++++++++++++++++++++++
> >  drivers/usb/host/xhci.h      |  2 ++
> >  include/linux/usb.h          |  2 ++
> >  4 files changed, 47 insertions(+)
> > 
> > diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> > index 40fa25c4d041..dd9805fb0566 100644
> > --- a/drivers/usb/host/xhci-ring.c
> > +++ b/drivers/usb/host/xhci-ring.c
> > @@ -3272,6 +3272,12 @@ int xhci_queue_bulk_tx(struct xhci_hcd *xhci, gfp_t
> > mem_flags,
> >  			field |= TRB_IOC;
> >  			more_trbs_coming = false;
> >  			td->last_trb = ring->enqueue;
> > +
> > +			if (urb->transfer_flags & URB_NO_DMA_MAP) {
> 
> do you really need the flag? Why don't you do this unconditionally as
> long as urb->transfer_length is <= 8?

There is a set of limitations, see my comments below.

> 
> > +				memcpy(&send_addr, urb->transfer_buffer,
> > +				       full_len);
> > +				field |= TRB_IDT;
> > +			}
> >  		}
> >  
> >  		/* Only set interrupt on short packet for IN endpoints */
> > diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
> > index 005e65922608..ce3b6663f940 100644
> > --- a/drivers/usb/host/xhci.c
> > +++ b/drivers/usb/host/xhci.c
> > @@ -1238,6 +1238,41 @@ EXPORT_SYMBOL_GPL(xhci_resume);
> >  
> >  /*-------------------------------------------------------------------------
> > */
> >  
> > +static void xhci_unmap_urb_for_dma(struct usb_hcd *hcd, struct urb *urb)
> > +{
> > +	if (urb->transfer_flags & URB_NO_DMA_MAP)
> > +		urb->transfer_flags &= ~URB_NO_DMA_MAP;
> > +	else
> > +		usb_hcd_unmap_urb_for_dma(hcd, urb);
> > +}
> > +
> > +static int xhci_map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
> > +				gfp_t mem_flags)
> > +{
> > +	int maxp = usb_endpoint_maxp(&urb->ep->desc);
> > +	int len = urb->transfer_buffer_length;
> > +	int ret = 0;
> > +
> > +	/*
> > +	 * Checks if URB is suitable for Immediate Transfer (IDT): instead of
> > +	 * mapping the buffer for DMA and passing the address to the host
> > +	 * controller, we copy the actual data into the TRB address register.
> > +	 * This is limited to transfers up to 8 bytes.
> > +	 *
> > +	 * IDT is only supported for Bulk and Interrupt endpoints. Apart from
> > +	 * the size constraints special care is taken to avoid cases where
> > +	 * wMaxPacketSize is smaller than 8 bytes as it's not supported.
> > +	 */
> > +	if ((usb_endpoint_is_int_out(&urb->ep->desc) ||
> > +	    usb_endpoint_is_bulk_out(&urb->ep->desc)) &&
> 
> I don't understand the check for endpoint type. IDT is, actually,
> already used for control endpoints because setup packets are composed of
> 8 bytes. You're also showing that this works for INT and BULK types. It
> would be a surprise if it doesn't work for ISOC.

Isoc should work, but I've been unable to find a device with such small
transfer size. As the standard says if IDT is set only a Transfer/Isoc TRB is
allowed per TD (see Table 6-22 on the spec). So I guess it makes it unlikely
for an Isoc transfer to match the constraints. That said I'll have a more in
depth look at it.

> 
> > +	    maxp >= TRB_IDT_MAX_SIZE && len <= TRB_IDT_MAX_SIZE)
> 
> why the maxp check? What if I'm an interrupt endpoint with maxp of 2?
> Is there really a limitation that you couldn't use IDT for those?

Same table in specs. As I state in the patch's comment above, IDT with
wMaxPacketSize < 8 it's not supported.

> 
> > +		urb->transfer_flags |= URB_NO_DMA_MAP;
> 
> do we really need this? I wonder if returning zero here would be enough,
> then you could:

I agree that with Isoc included we could lose the flag and implement it as you
state below. Only length and ep's wMaxPacketSize should be checked.

> 
> ...map_urb_for_dma(...)
> {
> 	ret = 0;
> 
> 	if (urb->transfer_length > 8)
>         	ret = usb_hcd_map_urb_for_dma(hcd, urb, flags);
> 
> 	return ret;
> }
> 
> ...unmap_urb_for_dma(...)
> {
> 	if urb->transfer_length > 8)
>         	usb_hcd_unmap_urb_for_dma(hcd, urb);
> }
> 
> ...xhci_queue_bulk_tx(...)
> {
> 	[...]
> 
> 	if (urb->transfer_length <= 8) {
> 		memcpy(&addr, urb->transfer_buffer, 8)l
> 		field |= TRB_IDT;
>         }
> 
> 	[...]
> }
> 

Regards,
Nicolas

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