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Date:   Wed, 20 Feb 2019 11:30:28 +0200
From:   Felipe Balbi <>
To:     Nicolas Saenz Julienne <>,
        Mathias Nyman <>
Cc:, Nicolas Saenz Julienne <>,
        Greg Kroah-Hartman <>,,
Subject: Re: [PATCH] usb: xhci: add Immediate Data Transfer support

Nicolas Saenz Julienne <> writes:

> 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 <>

This looks good to my eyes.

Reviewed-by: Felipe Balbi <>


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