linux-kernel - Re: [PATCH] usb: xhci: add Immediate Data Transfer support

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Message-ID: <1fe79a10-16d4-8d06-4c9b-7e5a3d3807cf@linux.intel.com>
Date:   Tue, 19 Mar 2019 10:15:18 +0200
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        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 15.3.2019 14.51, Nicolas Saenz Julienne wrote:
> 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? :)
> 

Looks good, adding to queue, thanks

-Mathias