Re: MMCv4 support (8-bit support missing)

[Philip Langdale]

Madhusudhan c wrote:
> Hi Pierre/philip,
> 
> This is regarding the MMCv4 support that came in as part of the MMC
> core of 2.6.20 linux kernel version.
> 
> The high speed MMC cards can support 4-bit/8-bit transfers. The 8-bit
> support seems to be missing from the MMCv4 support implemented by
> Philip Langdale .
> To support 8-bit transfers the core needs to implement "bus testing
> procedure". This requires support for CMD19 and CMD14.
> 
> Why is the support for bus testing procedure msiing from the MMCv4 support?

Because we have not seen any 8-bit hardware - let alone any 8-bit hardware
which we know how to drive. It is unclear how necessary the bus testing sequence
is, but the main reason why it's not there is that I could never get it to work
on the sdhci controller I use for testing. Due to how the CRC is handled for these
commands, the controller gets confused, reports failure, and doesn't provide the
returned data - making the test impossible.

If you have found an 8-bit controller that we can support, we'd love to know about
it!

--phil
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