Re: [PATCH-for-10.1 1/2] hw/sd/ssi-sd: Return noise (dummy byte) when no card connected

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 12/8/25 15:20, Alex Bennée wrote:
> Philippe Mathieu-Daudé <philmd@linaro.org> writes:
> 
>> Commit 1585ab9f1ba ("hw/sd/sdcard: Fill SPI response bits in card
>> code") exposed a bug in the SPI adapter: if no SD card is plugged,
>> we shouldn't return any particular packet response, but the noise
>> shifted on the MISO line. Return the dummy byte, otherwise we get:
>>
>>    qemu-system-riscv64: ../hw/sd/ssi-sd.c:160: ssi_sd_transfer: Assertion `s->arglen > 0' failed.
>>
>> Reported-by: Guenter Roeck <linux@roeck-us.net>
>> Fixes: 775616c3ae8 ("Partial SD card SPI mode support")
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>> ---
>>   hw/sd/ssi-sd.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/hw/sd/ssi-sd.c b/hw/sd/ssi-sd.c
>> index 594dead19ee..3aacbd03871 100644
>> --- a/hw/sd/ssi-sd.c
>> +++ b/hw/sd/ssi-sd.c
>> @@ -89,6 +89,10 @@ static uint32_t ssi_sd_transfer(SSIPeripheral *dev, uint32_t val)
>>       SDRequest request;
>>       uint8_t longresp[5];
>>   
>> +    if (!sdbus_get_inserted(&s->sdbus)) {
>> +        return SSI_DUMMY;
>> +    }
>> +
> 
> Seems fair although it's hard to track what is consuming this value. I
> think we end up in ssi_transfer() which a surprising number of calls
> don't even bother checking the return value, other just seem to | the
> result when iterating across devices.

A SPI transaction consists of shifting bit in sync the CLK line,
writing on the MOSI (output) line / and reading MISO (input) line.

IOW, each time you write a word, you also read it at the same time.

When a driver just wants to write, it is OK to ignore the returned
values.

In this case, we don't want to return "There is a card with error"
because there is no card. We shift the request on the MOSI line,
but nothing replies on the MISO line.

Should I reword the commit description?

> 
> We should probably improve on the definitions of transfer/transfer_raw
> and explain what the return value is.

Agreed.

> 
> Anyway:
> 
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

Thanks!