Jun 24 15:28:10 sheevaplug-debian kernel: btmrvl_sdio mmc0:0001:2: firmware: requesting sd8688_helper.bin
Jun 24 15:28:10 sheevaplug-debian kernel: btmrvl_sdio_download_helper: request_firmware(helper) failed, error code = -2
Jun 24 15:28:10 sheevaplug-debian kernel: btmrvl_sdio_download_fw: Failed to download helper!
Jun 24 15:28:10 sheevaplug-debian kernel: btmrvl_sdio_probe: Downloading firmware failed!
In the folder /lib/firmware/mrvl : i have helper_sd.bin sd8688_ap.bin sd8688.bin sd8688_helper.bin
What does mean the error code 2 ?
The error code 2 is ENOENT, which in this context means "file not found".
If you read the error message carefully, especially the part "requesting sd8688_helper.bin", you may notice that it wasn't requesting "mrvl/sd8688_helper.bin". The files "sd8688.bin" and "sd8688_helper.bin" should not be inside "mrvl", they should be directly in "/lib/firmware".
P.S. By the way, don't know if anyone noticed, the files "helper_sd.bin" and "sd8688_helper.bin" are exactly the same, so one of them can be replaced with a link to the other.