Re: [PATCH 2/2] spi: spi-mem: fallback to using transfers when CS gpios are used

[Chris Packham]

Hi Connor,

On 28/03/2026 07:29, Conor Dooley wrote:
> On Thu, Nov 07, 2019 at 05:42:35PM +1300, Chris Packham wrote:
>> Devices with chip selects driven via GPIO are not compatible with the
>> spi-mem operations. Fallback to using standard spi transfers when the
>> device is connected with a gpio CS.
> Bitta necromancy, but why are they not compatible?
> Couldn't you have just done an if/else in your driver to call
> gpiod_set_value(spi_get_csgpiod())?
>
> For some context, I've been trying to resolve some issues with using
> the standard transfers code when used with flash devices in the
> microchip-core-qspi driver, but if I delete this !mem->spi->cs_gpiod
> check, and add the aforementioned if/else in my driver, the mem ops
> work fine.
>
> What am I missing that makes them incompatible?

The cover letter from this series[1] adds a little context. Basically 
the hardware design I was dealing with is a bit odd. It doesn't wire up 
GPIOs as chip selects directly it uses a GPIO and some extra logic to 
steer the native CS. On the BCM58525 the CS appears to be driven 
automatically (other SoCs that use the bcm-qspi driver appear to be able 
to explicitly control the chip select). This combined with the ability 
to offload the various spi-mem operations means we need the spi core to 
drive the GPIO CS and use the standard spi operations.

I think that check could probably be replaced with some flag that 
amounts to disabling the spi-mem ops when we don't have full control 
over the native CS.

[1] - 
https://lore.kernel.org/all/20191107044235.4864-1-chris.packham@xxxxxxxxxxxxxxxxxxx/

>
> Cheers,
> Conor.
>
>> Signed-off-by: Chris Packham <chris.packham@xxxxxxxxxxxxxxxxxxx>
>> ---
>>   drivers/spi/spi-mem.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
>> index 9f0fa9f3116d..e5a46f0eb93b 100644
>> --- a/drivers/spi/spi-mem.c
>> +++ b/drivers/spi/spi-mem.c
>> @@ -286,7 +286,7 @@ int spi_mem_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
>>   	if (!spi_mem_internal_supports_op(mem, op))
>>   		return -ENOTSUPP;
>>   
>> -	if (ctlr->mem_ops) {
>> +	if (ctlr->mem_ops && !mem->spi->cs_gpiod) {
>>   		ret = spi_mem_access_start(mem);
>>   		if (ret)
>>   			return ret;
>> -- 
>> 2.24.0
>>