[U-Boot] [PATCH 1/2] spi: ti_qspi: Add dummy readl for bus sync

[Vignesh R <vigneshr@ti.com>]

On 11/17/2015 11:33 AM, Jagan Teki wrote:
> On 10 November 2015 at 11:52, Vignesh R <vigneshr@ti.com> wrote:
>> Add dummy readl after invalidating cmd field of QSPI_CMD_REG to ensure
>> bus sync. Without this device's CS is not deactivated reliably leading
>> to failure to enumerate flash or failure to set quad enable bit on
>> Macronix flash present on am437x-sk and am437x-idk evms.
>>
>> Signed-off-by: Vignesh R <vigneshr@ti.com>
>> ---
>>  drivers/spi/ti_qspi.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/spi/ti_qspi.c b/drivers/spi/ti_qspi.c
>> index bd63db8a2acc..ba2da017c21a 100644
>> --- a/drivers/spi/ti_qspi.c
>> +++ b/drivers/spi/ti_qspi.c
>> @@ -170,6 +170,8 @@ void spi_cs_deactivate(struct spi_slave *slave)
>>         debug("spi_cs_deactivate: 0x%08x\n", (u32)slave);
>>
>>         writel(qslave->cmd | QSPI_INVAL, &qslave->base->cmd);
>> +       /* dummy readl to ensure bus sync */
>> +       readl(&qslave->base->cmd);
> 
> Isn't effect anything for non-quad case? and dummy bytes might changed
> based on quad and quadio does this single dummy readl is same for
> those cases as well?

No, this dummy readl is not related as dummy bytes used in QUAD mode.

In ti_qspi controller, in order to end a read/write request (ie to
deassert ChipSelect) invalid cmd(0x4) needs to be written to CMD field
of QSPI_CMD_REG (which is done by above function) in all modes.
Sometimes, this write may have been flushed out of ARM but may not have
reached the QSPI IP yet (ie bus/interconnect is not flushed yet) hence
ChipSelect is not deactivated. Therefore, the successive write/read
operation may overlaps resulting in wrong data being read from flash.
Reading back QSPI_CMD_REG makes sure that the bus/interconnect is
flushed and register write is affected making sure CS is deactivated.

-- 
Regards
Vignesh