• Eric Lapuyade's avatar
    NFC: NCI: Modify NCI SPI to implement CS/INT handshake per the spec · 2bed2785
    Eric Lapuyade authored
    The NFC Forum NCI specification defines both a hardware and software
    protocol when using a SPI physical transport to connect an NFC NCI
    Chipset. The hardware requirement is that, after having raised the chip
    select line, the SPI driver must wait for an INT line from the NFC
    chipset to raise before it sends the data. The chip select must be
    raised first though, because this is the signal that the NFC chipset
    will detect to wake up and then raise its INT line. If the INT line
    doesn't raise in a timely fashion, the SPI driver should abort
    operation.
    
    When data is transferred from Device host (DH) to NFC Controller (NFCC),
    the signaling sequence is the following:
    
    Data Transfer from DH to NFCC
    • 1-Master asserts SPI_CSN
    • 2-Slave asserts SPI_INT
    • 3-Master sends NCI-over-SPI protocol header and payload data
    • 4-Slave deasserts SPI_INT
    • 5-Master deasserts SPI_CSN
    
    When data must be transferred from NFCC to DH, things are a little bit
    different.
    
    Data Transfer from NFCC to DH
    • 1-Slave asserts SPI_INT -> NFC chipset irq handler called -> process
    reading from SPI
    • 2-Master asserts SPI_CSN
    • 3-Master send 2-octet NCI-over-SPI protocol header
    • 4-Slave sends 2-octet NCI-over-SPI protocol payload length
    • 5-Slave sends NCI-over-SPI protocol payload
    • 6-Master deasserts SPI_CSN
    
    In this case, SPI driver should function normally as it does today. Note
    that the INT line can and will be lowered anytime between beginning of
    step 3 and end of step 5. A low INT is therefore valid after chip select
    has been raised.
    
    This would be easily implemented in a single driver. Unfortunately, we
    don't write the SPI driver and I had to imagine some workaround trick to
    get the SPI and NFC drivers to work in a synchronized fashion. The trick
    is the following:
    
    - send an empty spi message: this will raise the chip select line, and
    send nothing. We expect the /CS line will stay arisen because we asked
    for it in the spi_transfer cs_change field
    - wait for a completion, that will be completed by the NFC driver IRQ
    handler when it knows we are in the process of sending data (NFC spec
    says that we use SPI in a half duplex mode, so we are either sending or
    receiving).
    - when completed, proceed with the normal data send.
    
    This has been tested and verified to work very consistently on a Nexus
    10 (spi-s3c64xx driver). It may not work the same with other spi
    drivers.
    
    The previously defined nci_spi_ops{} whose intended purpose were to
    address this problem are not used anymore and therefore totally removed.
    
    The nci_spi_send() takes a new optional write_handshake_completion
    completion pointer. If non NULL, the nci spi layer will run the above
    trick when sending data to the NFC Chip. If NULL, the data is sent
    normally all at once and it is then the NFC driver responsibility to
    know what it's doing.
    Signed-off-by: default avatarEric Lapuyade <eric.lapuyade@intel.com>
    Signed-off-by: default avatarSamuel Ortiz <sameo@linux.intel.com>
    2bed2785
spi.c 7.33 KB