[U-Boot] [PATCH] mmc: Do not poll using CMD13 when changing timing

Marek Vasut marek.vasut at gmail.com
Tue Jun 25 19:47:00 UTC 2019

On 6/25/19 9:07 PM, Jean-Jacques Hiblot wrote:
> On 25/06/2019 17:53, Marek Vasut wrote:
>> On 6/25/19 4:13 PM, Jean-Jacques Hiblot wrote:
>>> Hi Marek,
>>> On 06/02/2019 14:11, Marek Vasut wrote:
>>>> When using CMD6 to switch eMMC card timing from HS200/HS400 to
>>>> HS/legacy,
>>>> do not poll for the completion status using CMD13, but rather wait
>>>> 50mS.
>>>> Once the card receives the CMD6 and starts executing it, the bus is in
>>>> undefined state until both the card finishes executing the command and
>>>> until the controller switches the bus to matching timing configuration.
>>>> During this time, it is not possible to transport any commands or data
>>>> across the bus, which includes the CMD13.
>>> Is it something that affects all platforms ?
>>> I've been working on this on TI platforms lately and although I've seen
>>> the same kind of problems. But they mostly go away If the code handling
>>> the switch handles the SWITCH_ERROR bit.
>> The error probably happens because you send the card a command, but the
>> command hits the bus that's in some weird intermediate state. Either the
>> card didn't finish switching the bus properties yet, or it did, or it's
>> in the middle of it, but you cannot tell.
> OK thanks for the response.
> If CMD13 can't be used because the bus is in an undefined state,
> shouldn't send_cmd(CMD13) just fail ?

So it would fail, but how do you guarantee the bus/card switched to the
new state and is reliably ready ? You can't, because you cannot
communicate with the card -- which is the entire problem here, you have
no way to communicate with the card while the bus is switching. However,
you can wait the maximum allowed time according to the spec, and then
the bus/card must be ready. If not, all bets are off.

> I'm asking because this is a 50ms delay in the hot path of the boot
> process.

I totally understand that, I have the same issue with HS200/HS400 on
R-Car Gen3, all the calibration shenanigans and this indeed add quite a
bit to the boot time. But I don't have a good answer here.

Best regards,
Marek Vasut

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