[ata] wait_for_bsy vs. wait_for_rdy: ATA spec interpretation

Hi folks,

(heh, I hope this won't be too long, but its essentially just a short
question with a „bit“ of context)

I'm still messing with ATA code to debug further SSD issues. Now I
stumbled across what rockbox code does before command issuing. Basically
its like this:

(1) Select the device
(2) wait_for_rdy
(2a) wait_for_bsy (or return with error)
(2a.i) wait until STATUS_BSY is cleared (or timeout)
(2a) wait until STATUS_RDY is set (or timeout)
(3) Setup parameters and issue the command

(see firmware/ata.c, searching for ATA_SELECT for reference, its done at
several places)

However, ATA spec draft 5 (section 9.3, its at page 236 in the version I
have) has the following state graph for the bus idle mode (i.e. before a
command is being executed):

Check_Status:
    BSY = 0 & DRQ = 0 & wrong device selected
      => Device_Select
    BSY = 0 & DRQ = 0 & correct device selected
      => Write_parameters
    BSY = 1 or DRQ = 1
      => Check_Status