Posted on May 17, 2011 at 14:02Where the problem is depends on what code you're using, of course. I'm using FWLib version 2.0.3, which is not the latest. I haven't looked at FWLib V3 in detail yet but I believe the SDIO stuff it is pretty similar. Most of the ELM/SDIO interface code I've seen is lifted more-or-less verbatim from the FWLib SDIO example code.
To use ELM, you must implement a disk_read() function, which has a ''BYTE *'' parameter for the destination buffer. This is the parameter which can be misaligned. If you blindly pass that parameter on to SD_ReadBlock(), and you're using DMA, then that pointer is probably being cast to a ''u32 *'' pointer. In my implementation, that pointer got passed to ''DMA_Configuration()''; but that function expects an aligned address (in my code).
My work-around was to inspect the pointer, and only if it is mis-aligned, then use a local, aligned buffer for ''SD_ReadBlock()'' instead. After the read completes, I copy the data from that local buffer to the caller's desired location before returning (taking a performance hit). A bit more jiggery-pokery is required to make the SD_ReadMultiBlocks() do the right thing; and similar work must be done on the Write functions. I'm in the process of testing my changes.
The ELM code calls ''disk_read()'' to read FAT entries as well as user file data directly from the SD card. For short, small ''f_read()'' calls, the corresponding ''disk_read()'' calls are almost always directed to the ELM's internal buffer, so no problem occurs. In this case, ELM extracts the partial-sector data that you asked for from its internal buffer, before returning from the ''f_read()'' call.
However, if (say) you are doing an ''f_read()'' call of a very large block of data, then that translates to reading of many sectors. When ALL of the sector-data in a ''disk_read()'' call is destined to be passed to the user (i.e. at least 512 bytes, and not FAT data), then ELM may bypass its internal buffer, and ask ''disk_read()'' to put the data directly into your buffer, as specified in your ''f_read()'' call.
Hope that helps.
