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SubjectRe: [PATCH] usb: xhci: add Immediate Data Transfer support
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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@suse.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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