Re: [PATCH v2 08/25] hw/sd: ssi-sd: Add a state representing Nac

[Philippe Mathieu-Daudé]

On 1/23/21 11:39 AM, Bin Meng wrote:
> From: Bin Meng <bin.meng@windriver.com>
> 
> Per the "Physical Layer Specification Version 8.00" chapter 7.5.2,
> "Data Read", there is a minimum 8 clock cycles (Nac) after the card
> response and before data block shows up on the data out line. This
> applies to both single and multiple block read operations.
> 
> Current implementation of single block read already satisfies the
> timing requirement as in the RESPONSE state after all responses are
> transferred the state remains unchanged. In the next 8 clock cycles
> it jumps to DATA_START state if data is ready.
> 
> However we need an explicit state when expanding our support to
> multiple block read in the future. Let's add a new state PREP_DATA
> explicitly in the ssi-sd state machine to represent Nac.
> 
> Note we don't change the single block read state machine to let it
> jump from RESPONSE state to DATA_START state as that effectively
> generates a 16 clock cycles Nac, which might not be safe. As the
> spec says the maximum Nac shall be calculated from several fields
> encoded in the CSD register, we don't want to bother updating CSD
> to ensure our Nac is within range to complicate things.

As I don't have access to that part of the spec, I'm going to trust you.

Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

> 
> Signed-off-by: Bin Meng <bin.meng@windriver.com>
> 
> ---
> 
> Changes in v2:
> - new patch: add a state representing Nac
> 
>  hw/sd/ssi-sd.c | 5 +++++
>  1 file changed, 5 insertions(+)