[U-Boot] [PATCH v2 2/5] ehci-hcd: Boost transfer speed

[Stefan Herbrechtsmeier]

Am 20.07.2012 17:03, schrieb Benoît Thébaudeau:
> On Friday 20 July 2012 16:51:33 Stefan Herbrechtsmeier wrote:
>> Am 20.07.2012 15:56, schrieb Benoît Thébaudeau:
>>> Dear Marek Vasut,
>>>
>>> On Friday 20 July 2012 15:44:01 Marek Vasut wrote:
>>>>> On Friday 20 July 2012 13:37:37 Stefan Herbrechtsmeier wrote:
>>>>>> Am 20.07.2012 13:26, schrieb Benoît Thébaudeau:
>>>>>>> +			int xfr_bytes = min(left_length,
>>>>>>> +					    (QT_BUFFER_CNT * 4096 -
>>>>>>> +					     ((uint32_t)buf_ptr & 4095)) &
>>>>>>> +					    ~4095);
>>>>>> Why you align the length to 4096?
>>>>> It's to guarantee that each transfer length is a multiple of the
>>>>> max packet
>>>>> length. Otherwise, early short packets are issued, which breaks
>>>>> the
>>>>> transfer and results in time-out error messages.
>>>> Early short packets ? What do you mean?
>>> During a USB transfer, all packets must have a length of max packet
>>> length for
>>> the pipe/endpoint, except the final one that can be a short packet.
>>> Without the
>>> alignment I make for xfr_bytes, short packets can occur within a
>>> transfer,
>>> because the hardware starts a new packet for each new queued qTD it
>>> handles.
>> But if I am right, the max packet length is 512 for bulk and 1024 for
>> Interrupt transfer.
> There are indeed different max packet lengths for different transfer types, but
> it does not matter since the chosen alignment guarantees a multiple of all these
> possible max packet lengths.
But thereby you limit the transfer to 4 qT buffers for unaligned transfers.

Best regards,
     Stefan