[U-Boot] [PATCH] usb_storage: fix ehci driver max transfer size

[Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>]

Am 04.07.2012 08:57, schrieb Schneider, Kolja:
>> Am 03.07.2012 20:10, schrieb Marek Vasut:
>>>> The commit 5dd95cf93dfffa1d19a1928990852aac9f55b9d9 'usb_storage:
>>>> Fix EHCI "out of buffer pointers" with CD-ROM' introduce a bug in
>>>> usb_storage as it wrongly assumes that every transfer can use 4k
>>>> per qt_buffer. This is wrong if the start address of the data
>>>> is not 4k aligned and leads to 'EHCI timed out on TD' messages
>>>> because of 'out of buffer pointers' in ehci_td_buffer function.
>>>>
>>>> Cc: Marek Vasut <marex@denx.de>
>>>> Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>
>>> Ok, first I have to admit I broke my promise to look into this ASAP, sorry
>> about
>>> it :-(
>> No problem, as long as we get it into the next release. ;-)
>>> Just curious, but shouldn't it be ((4096 * 5) / dev_desc->blk_sz) - 1 ?
>> No, because the first blk need to be aligned with the 4096. In worst
>> case the blk is at the end of the 4096 range. If we assume that the blk
>> is aligned to blk_sz we can change it to ((4096 * 4) / dev_desc->blk_sz)
>> + 1. I skip the last blk (+ 1) because with 4096 aligned first blk we
>> unaligned the next transfer and add extra short packages to each ehci
>> transfer.
>>
>> If we want to maximise the usage we need to calculate the max_xfer_blk
>> depending on the start address of the first blk.
>>
> I admit to not totally getting it. However, there are two things that come to my mind:
>   - 	Doesn't the EHCI Specification mention exactly five buffers that can/should/must
>     	be used?
Yes, you can use up to five 4096 byte buffers.
>   - 	I think I once stumbled across some comment that said as much as the blocks
> 	always having to be aligned anyway?
The buffers must be aligned to a 4096 byte page. This means that you 
have to use the first and last buffer to align your data to the next or 
previous 4096 byte page boundary.