spi transfer with zero length

[Atsushi Nemoto]

Hi.  Is it legal to use zero for 'len' field of struct spi_transfer?
I mean, len=0, tx_buf=rx_buf=NULL, delay_usecs!=0.

Some SPI devices need slightly long delay before first CLK edge after
CS assertion.  To achieve this, I think inserting using a zero length
transfer before real transfers.  But it seems some drivers do not
handle this case properly.

Is this driver's bug, or we need additional delay field in struct
spi_device for such case?

---
Atsushi Nemoto
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