linux-kernel - Re: [PATCH v3 3/4] spi: spi-geni-qcom: Don't try to set CS if an xfer is pending

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Date:   Thu, 17 Dec 2020 18:54:23 -0800
From:   Stephen Boyd <swboyd@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>,
        Mark Brown <broonie@...nel.org>
Cc:     msavaliy@....qualcomm.com, akashast@...eaurora.org,
        Roja Rani Yarubandi <rojay@...eaurora.org>,
        Douglas Anderson <dianders@...omium.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org
Subject: Re: [PATCH v3 3/4] spi: spi-geni-qcom: Don't try to set CS if an xfer is pending

Quoting Douglas Anderson (2020-12-17 14:29:13)
> If we get a timeout sending then this happens:
> 
> spi_transfer_one_message()
>  ->transfer_one() AKA spi_geni_transfer_one()
>   setup_fifo_xfer()
>    mas->cur_xfer = non-NULL
>  spi_transfer_wait() => TIMES OUT
>  if (msg->status != -EINPROGRESS)
>   goto out
>  if (ret != 0 ...)
>   spi_set_cs()
>    ->set_cs AKA spi_geni_set_cs()
>     # mas->cur_xfer is non-NULL
> 
> The above happens _before_ the SPI core calls ->handle_err() AKA
> handle_fifo_timeout().
> 
> Unfortunately that won't work so well on geni.  If we got a timeout
> transferring then it's likely that our interrupt handler is blocked,
> but we need that same interrupt handler to run and the command channel
> to be unblocked in order to adjust the chip select.  Trying to set the
> chip select doesn't crash us but ends up confusing our state machine
> and leads to messages like: Premature done. rx_rem = 32 bpw8
> 
> Let's just drop the chip select request in this case.  We can detect
> the case because cur_xfer is non-NULL--it would have been set to NULL
> in the interrupt handler if the previous transfer had finished.  Sure,
> we might leave the chip select in the wrong state but it's likely it
> was going to fail anyway and this avoids getting the driver even more
> confused about what it's doing.
> 
> The SPI core in general assumes that setting chip select is a simple
> operation that doesn't fail.  Yet another reason to just reconfigure
> the chip select line as GPIOs.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---

Reviewed-by: Stephen Boyd <swboyd@...omium.org>