[U-Boot] MMC and buffer alignment question

Eric Nelson
2014-03-19T18:18:02Z

Hi Markus and Wolfgang,

On 03/19/2014 10:40 AM, Markus Niebel wrote:
> Am 19.03.2014 15:44, wrote Wolfgang Denk:
>> Dear Eric Nelson,
>>
>> In message <5329A80B.9020908@boundarydevices.com> you wrote:
>>>
>>>> short question to the usage of the mmc command (and also the mmc
>>>> driver API): is it intended that mmc read / write may fail when the
>>>> supplied address in RAM is not aligned?
>>>
>>> If not intended, it is known.
>>
>> I consider this a known bug.
>>
>>>> ARMV7 will give output like this:
>>>>
>>>> U-Boot > mmc read 12000002 44 44
>>>>
>>> Why would you want to do this?
>>
>> For example, BMP images require loading on a +2 aligned address due to
>> their stupid header format. I ran into this before myself: it is
>> impossible to match both the alignment reuqirements of the bmp command
>> and the mmc read command at the same time. One must manually copy the
>> memory ragen again. This is a plain, stupid bug.
>>
>
> Exactly here it popped up ...
>

It seems to me that if you're resorting to using un-formatted
storage space to store a broken data structure (the BMP header),
you could just write it at an offset +2.

The BMP support is pretty difficult to use anyway (only supports
BMPV3 headers), so asking the user to know about the offset doesn't
seem onerous.

Also note that the patch I submitted recently handles the case
for gzipped files for those using cfb_console.

>>>> Special commands inside the mmc drivers and in env_mmc implement the
>>>> alignment magic. Shouldn't the mmc do the magic (and if neccesarry
>>>> provide help using temp buffers if needed) so that all users outside can
>>>> read / write without caring for special cases?
>>>
>>> Is there a use case here? There are plenty of memory addresses that
>>> won't work with commands like "mmc read".
>>
>
> env_mmc needs to care for cache aligned buffers - This was fixed some time ago
> for the redundant env case
>
>> "mmc read" and "mmc write" are operations that work on character
>> buffers, like all other file IO ops. These should not require any
>> specific alignment.
>>
>>> Is it worth **any** code to try and catch them?
>>
>> Definitely yes.
>>

IMHO, this seems like overkill. Should we also prevent over-writing
the stack or heap?

>
> So just as an idea - we could use a bounce buffer for mmc_bwrite / mmc_bread for the
> unaligned case. Is definitely slow but should work.
>

Note that "sata read/write" and "usb read/write" have the same issues.

Regards,

Eric