Re: [PATCH v4 08/11] spi: aspeed: Calibrate read timings

["Cédric Le Goater" <clg@kaod.org>]

On 3/31/22 18:41, Pratyush Yadav wrote:
> Hi,
> 
> On 25/03/22 11:08AM, Cédric Le Goater wrote:
>> To accommodate the different response time of SPI transfers on different
>> boards and different SPI NOR devices, the Aspeed controllers provide a
>> set of Read Timing Compensation registers to tune the timing delays
>> depending on the frequency being used. The AST2600 SoC has one of these
>> registers per device. On the AST2500 and AST2400 SoCs, the timing
>> register is shared by all devices which is problematic to get good
>> results other than for one device.
>>
>> The algorithm first reads a golden buffer at low speed and then performs
>> reads with different clocks and delay cycle settings to find a breaking
>> point. This selects a default good frequency for the CEx control register.
>> The current settings are a bit optimistic as we pick the first delay giving
>> good results. A safer approach would be to determine an interval and
>> choose the middle value.
>>
>> Calibration is performed when the direct mapping for reads is created.
>> Since the underlying spi-nor object needs to be initialized to create
>> the spi_mem operation for direct mapping, we should be fine. Having a
>> specific API would clarify the requirements though.
>>
>> Cc: Pratyush Yadav <p.yadav@ti.com>
>> Reviewed-by: Joel Stanley <joel@jms.id.au>
>> Tested-by: Joel Stanley <joel@jms.id.au>
>> Tested-by: Tao Ren <rentao.bupt@gmail.com>
>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
>> ---
>>   drivers/spi/spi-aspeed-smc.c | 281 +++++++++++++++++++++++++++++++++++
>>   1 file changed, 281 insertions(+)
>>
> [...]
>> @@ -517,6 +527,8 @@ static int aspeed_spi_chip_adjust_window(struct aspeed_spi_chip *chip,
>>   	return 0;
>>   }
>>   
>> +static int aspeed_spi_do_calibration(struct aspeed_spi_chip *chip);
>> +
>>   static int aspeed_spi_dirmap_create(struct spi_mem_dirmap_desc *desc)
>>   {
>>   	struct aspeed_spi *aspi = spi_controller_get_devdata(desc->mem->spi->master);
>> @@ -565,6 +577,8 @@ static int aspeed_spi_dirmap_create(struct spi_mem_dirmap_desc *desc)
>>   	chip->ctl_val[ASPEED_SPI_READ] = ctl_val;
>>   	writel(chip->ctl_val[ASPEED_SPI_READ], chip->ctl);
>>   
>> +	ret = aspeed_spi_do_calibration(chip);
>> +
> 
> I am still not convinced this is a good idea. The API does not say
> anywhere what dirmap_create must be called after the flash is completely
> initialized, though that is what is done currently in practice. 

Yes because we wouldn't have a correct 'spi_mem_dirmap_info' if it wasn't
the case. May be change the documentation ?

> I think
> an explicit API to mark flash as "ready for calibration" would be a
> better idea.

OK. Since the above is a oneliner, it should not be a problem to move
it under a new handler if needed.

The dirmap_create() handler expects the spi-mem descriptor and the field
'desc->info.op_tmpl' to be correctly initialized in order to compute the
control register value, which is a requirement for dirmap_read(). The
calibration sequence simply comes after.

AFAICT, there is nothing incorrect today.

> Tudor/Mark/Miquel, what do you think?


Thanks,

C.