in l3gd20 driver startup, there is a setup failed error return from
stm32 spi driver

     "
     [    2.687630] st-gyro-spi spi0.0: supply vdd not found, using dummy
     regulator
     [    2.696869] st-gyro-spi spi0.0: supply vddio not found, using dummy
     regulator
     [    2.706707] spi_stm32 40015000.spi: SPI transfer setup failed
     [    2.713741] st-gyro-spi spi0.0: SPI transfer failed: -22
     [    2.721096] spi_master spi0: failed to transfer one message from queue
     [    2.729268] iio iio:device0: failed to read Who-Am-I register.
     [    2.737504] st-gyro-spi: probe of spi0.0 failed with error -22
     "

after debug into spi-stm32 driver, st-gyro-spi split two steps to read
l3gd20 id

first: send command to l3gd20 with read id command in tx_buf, rx_buf
is null.
second: read id with tx_buf is null, rx_buf not null.

so, for second step, stm32 driver recongise this process as 'SPI_SIMPLE_RX'
from stm32_spi_communication_type(), but there is no related process for this
type in stm32f4_spi_set_mode(), then we get error from
stm32_spi_transfer_one_setup().

we can use two method to fix this bug.
1, use stm32 spi's "In unidirectional receive-only mode (BIDIMODE=0 and
RXONLY=1)". but as our code running in sdram, the read latency is too large
to get so many receive overrun error in interrupts handler.

2, use stm32 spi's "In full-duplex (BIDIMODE=0 and RXONLY=0)", as tx_buf is
null, so add flag 'SPI_MASTER_MUST_TX' to spi master.

Change since V4:
1 remove dummy data sent out by stm32 spi driver
2 add flag 'SPI_MASTER_MUST_TX' to spi master
Signed-off-by: dillon min <dillon.minfei@gmail.com>
Link: https://lore.kernel.org/r/1590378348-8115-8-git-send-email-dillon.minfei@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>