[U-Boot] [PATCH v1 06/11] mmc: if possible, poll the busy state using DAT0

[Jean-Jacques Hiblot]

On 28/06/2019 15:38, Marek Vasut wrote:
> On 6/28/19 10:28 AM, Jean-Jacques Hiblot wrote:
>> On 28/06/2019 02:07, Marek Vasut wrote:
>>> On 6/27/19 12:31 PM, Jean-Jacques Hiblot wrote:
>>>> Using the DAT0 line as a rdy/busy line is an alternative to reading the
>>>> status register of the card. It especially useful in situation where the
>>>> bus is not in a good shape, like when modes are switched.
>>>> This is also how the linux driver behaves.
>>>>
>>>> Signed-off-by: Jean-Jacques Hiblot <jjhiblot at ti.com>
>>> Is this documented somewhere that this is an OK thing to do ?
>>> If the bus is not in a good shape, what guarantee do we have that DAT0
>>> (part of the bus ;-) ) would be in good shape ?
>> The spec states in "6.6.1 Command sets and extended settings": [...] The
>> SWITCH command response is of type R1b, therefore, the host should read
>> the Device status, using SEND_STATUS command, after the busy signal is
>> de-asserted, to check the result of the SWITCH operation.[...]
>>
>> Also this is how it is done in linux, except that instead of
>> wait_dat0(), the host driver provides a card_busy() function.
>>
>> I think that "not in good shape" probably means transients on the bus,
>> maybe due to changes in the voltage levels. Looking at DAT0 should be
>> fine because it is grounded while the card is busy.
>>
>> The only trouble with using DAT0, is that the clock must run while
>> waiting otherwise DAT0 may not be de-asserted, hence the encapsulation
>> in the wait_dat0() so that the HW can disable automatic clock-gating
>> before monitoring DAT0. Here is the relevant text from the spec:  6.7
>> "clock control"  [...] The host is allowed to shut down the clock of a
>> “busy” Device. The Device will complete the programming operation
>> regardless of the host clock. However, the host must provide a clock
>> edge for the Device to turn off its busy signal. Without a clock edge
>> the Device (unless previously disconnected by a deselect command (CMD7))
>> will force the DAT0 line down, forever. [...]
> Ah, good, maybe this should be in the commit message?
which part ? the clock stuff ?
>
> Also, which version of the spec is that ?
I'm quoting from JESD84-B51A (rev 5.1A) but IFAIK rdy/busy on DAT0 has 
been there from the start.
>