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Date:   Fri, 15 Mar 2019 13:51:09 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Mathias Nyman <mathias.nyman@...el.com>
Cc:     felipe.balbi@...ux.intel.com, oneukum@...e.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: xhci: add Immediate Data Transfer support

On Tue, 2019-02-19 at 17:29 +0100, Nicolas Saenz Julienne wrote:
> Immediate data transfers (IDT) allow the HCD to copy small chunks of
> data (up to 8bytes) directly into its output transfer TRBs. This avoids
> 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.
> 
> The implementation could cover all kind of output endpoints. Yet
> Isochronous endpoints are bypassed as I was unable to find one that
> matched IDT's constraints. As we try to bypass the default DMA mappings
> on URB buffers we'd need to find a Isochronous device with an
> urb->transfer_buffer_length <= 8 bytes.
> 
> The implementation takes into account that the 8 byte buffers provided
> by the URB will never cross a 64KB boundary.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>


Friendly ping, any more comments on this? :)

> ---
>  drivers/usb/host/xhci-ring.c | 12 ++++++++++++
>  drivers/usb/host/xhci.c      | 16 ++++++++++++++++
>  drivers/usb/host/xhci.h      | 17 +++++++++++++++++
>  3 files changed, 45 insertions(+)
> 
> diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> index 40fa25c4d041..997edc908a0d 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 (xhci_urb_suitable_for_idt(urb)) {
> +				memcpy(&send_addr, urb->transfer_buffer,
> +				       trb_buff_len);
> +				field |= TRB_IDT;
> +			}
>  		}
>  
>  		/* Only set interrupt on short packet for IN endpoints */
> @@ -3411,6 +3417,12 @@ int xhci_queue_ctrl_tx(struct xhci_hcd *xhci, gfp_t
> mem_flags,
>  	if (urb->transfer_buffer_length > 0) {
>  		u32 length_field, remainder;
>  
> +		if (xhci_urb_suitable_for_idt(urb)) {
> +			memcpy(&urb->transfer_dma, urb->transfer_buffer,
> +			       urb->transfer_buffer_length);
> +			field |= TRB_IDT;
> +		}
> +
>  		remainder = xhci_td_remainder(xhci, 0,
>  				urb->transfer_buffer_length,
>  				urb->transfer_buffer_length,
> diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
> index 005e65922608..f04ad2290884 100644
> --- a/drivers/usb/host/xhci.c
> +++ b/drivers/usb/host/xhci.c
> @@ -1238,6 +1238,21 @@ EXPORT_SYMBOL_GPL(xhci_resume);
>  
>  /*-------------------------------------------------------------------------*/
>  
> +/*
> + * Bypass the DMA mapping if URB is suitable for Immediate Transfer (IDT),
> + * we'll copy the actual data into the TRB address register. This is limited
> to
> + * transfers up to 8 bytes on output endpoints of any kind with
> wMaxPacketSize
> + * >= 8 bytes. If suitable for IDT only one Transfer TRB per TD is allowed.
> + */
> +static int xhci_map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
> +				gfp_t mem_flags)
> +{
> +	if (xhci_urb_suitable_for_idt(urb))
> +		return 0;
> +
> +	return usb_hcd_map_urb_for_dma(hcd, urb, mem_flags);
> +}
> +
>  /**
>   * xhci_get_endpoint_index - Used for passing endpoint bitmasks between the
> core and
>   * HCDs.  Find the index for an endpoint given its descriptor.  Use the
> return
> @@ -5155,6 +5170,7 @@ static const struct hc_driver xhci_hc_driver = {
>  	/*
>  	 * managing i/o requests and associated device resources
>  	 */
> +	.map_urb_for_dma =      xhci_map_urb_for_dma,
>  	.urb_enqueue =		xhci_urb_enqueue,
>  	.urb_dequeue =		xhci_urb_dequeue,
>  	.alloc_dev =		xhci_alloc_dev,
> diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
> index 652dc36e3012..7dc6d2197641 100644
> --- a/drivers/usb/host/xhci.h
> +++ b/drivers/usb/host/xhci.h
> @@ -1295,6 +1295,8 @@ enum xhci_setup_dev {
>  #define TRB_IOC			(1<<5)
>  /* The buffer pointer contains immediate data */
>  #define TRB_IDT			(1<<6)
> +/* TDs smaller than this might use IDT */
> +#define TRB_IDT_MAX_SIZE	8
>  
>  /* Block Event Interrupt */
>  #define	TRB_BEI			(1<<9)
> @@ -2141,6 +2143,21 @@ static inline struct xhci_ring
> *xhci_urb_to_transfer_ring(struct xhci_hcd *xhci,
>  					urb->stream_id);
>  }
>  
> +/*
> + * TODO: As per spec Isochronous IDT transmissions are supported. We bypass
> + * them anyways as we where unable to find a device that matches the
> + * constraints.
> + */
> +static inline bool xhci_urb_suitable_for_idt(struct urb *urb)
> +{
> +	if (!usb_endpoint_xfer_isoc(&urb->ep->desc) && usb_urb_dir_out(urb) &&
> +	    usb_endpoint_maxp(&urb->ep->desc) >= TRB_IDT_MAX_SIZE &&
> +	    urb->transfer_buffer_length <= TRB_IDT_MAX_SIZE)
> +		return true;
> +
> +	return false;
> +}
> +
>  static inline char *xhci_slot_state_string(u32 state)
>  {
>  	switch (state) {


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