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

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Message-ID: <20200601014646.GA794847@linux.intel.com>
Date:   Mon, 1 Jun 2020 04:47:07 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Peter Huewe <peterhuewe@....de>,
        Andrey Pronin <apronin@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tpm_tis_spi: Don't send anything during flow control

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@...omium.org>
> ---
> 
>  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