Re: [PATCH] tpm_tis_spi: Don't send anything during flow control

[Jarkko Sakkinen]

On Thu, May 28, 2020 at 03:19:30PM -0700, Douglas Anderson wrote:
> During flow control we are just reading from the TPM, yet our spi_xfer
> has the tx_buf and rx_buf both non-NULL which means we're requesting a
> full duplex transfer.
> 
> SPI is always somewhat of a full duplex protocol anyway and in theory
> the other side shouldn't really be looking at what we're sending it
> during flow control, but it's still a bit ugly to be sending some
> "random" data when we shouldn't.
> 
> The default tpm_tis_spi_flow_control() tries to address this by
> setting 'phy->iobuf[0] = 0'.  This partially avoids the problem of
> sending "random" data, but since our tx_buf and rx_buf both point to
> the same place I believe there is the potential of us sending the
> TPM's previous byte back to it if we hit the retry loop.
> 
> Another flow control implementation, cr50_spi_flow_control(), doesn't
> address this at all.
> 
> Let's clean this up and just make the tx_buf NULL before we call
> flow_control().  Not only does this ensure that we're not sending any
> "random" bytes but it also possibly could make the SPI controller
> behave in a slightly more optimal way.
> 
> NOTE: no actual observed problems are fixed by this patch--it's was
> just made based on code inspection.
> 
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> 
>  drivers/char/tpm/tpm_tis_spi_main.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm_tis_spi_main.c b/drivers/char/tpm/tpm_tis_spi_main.c
> index d96755935529..8d2c581a93c6 100644
> --- a/drivers/char/tpm/tpm_tis_spi_main.c
> +++ b/drivers/char/tpm/tpm_tis_spi_main.c
> @@ -53,8 +53,6 @@ static int tpm_tis_spi_flow_control(struct tpm_tis_spi_phy *phy,
>  
>  	if ((phy->iobuf[3] & 0x01) == 0) {
>  		// handle SPI wait states
> -		phy->iobuf[0] = 0;
> -

Why this should be removed?

/Jarkko