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

[Stefan Herbrechtsmeier]

Am 07.07.2012 23:58, schrieb Marek Vasut:
>> 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 at denx.de>
>>>>>> Signed-off-by: Stefan Herbrechtsmeier <stefan at 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.
> All right, I managed to replicate the issue. This (or similar) doesn't work for
> you, right:
>
> usb read 0x42000004 0x0 0x400
I observe the bug during
fatload usb 0 0x800000 uImage

...
dev=0ffb0440, pipe=c8008283, buffer=00a90000, length=20480, req=(null)
...
dev=0ffb0440, pipe=c8008283, buffer=00a95000, length=20480, req=(null)
...
dev=0ffb0440, pipe=c8008283, buffer=00a9a000, length=18432, req=(null)
...
dev=0ffb0440, pipe=c8008283, buffer=00a9e800, length=20480, req=(null)
out of buffer pointers (2048 bytes left)
...

It looks like the file is fragmented. During load the start address 
becomes unaligned and thereby the code wrongly tries to transfer more 
blocks than possible.

Before the above mentioned patch the max_xfer_blk was much smaller (20), 
so that the problem never appears.
> The proper solution would be to introduce a bounce buffer for such unaligned
> transfers. A proper, generic bounce buffer that can be configured to bounce on
> specified boundaries and warns about performance penalties.
Why not limit the max_xfer_blk to the maximum save count [(4096 * 4) / 
dev_desc->blksz)] or calculate the max_xfer_blk depending on the start 
address of the transfer:

max_xfer_blk = ((4096 * 5) - (start & ~4096) / dev_desc->blksz

Regards,
     Stefan