Re: [PATCH v3 1/2] arm64: dts: qcom: sc7180: Provide pinconf for SPI to use GPIO for CS

[Akash Asthana <akashast@codeaurora.org>]

On 9/22/2020 2:57 AM, Douglas Anderson wrote:
> When the chip select line is controlled by the QUP, changing CS is a
> time consuming operation.  We have to send a command over to the geni
> and wait for it to Ack us every time we want to change (both making it
> high and low).  To send this command we have to make a choice in
> software when we want to control the chip select, we have to either:
> A) Wait for the Ack via interrupt which slows down all SPI transfers
>     (and incurrs extra processing associated with interrupts).
> B) Sit in a loop and poll, waiting for the Ack.
>
> Neither A) nor B) is a great option.
>
> We can avoid all of this by realizing that, at least on some boards,
> there is no advantage of considering this line to be a geni line.
> While it's true that geni _can_ control the line, it's also true that
> the line can be a GPIO and there is no downside of viewing it that
> way.  Setting a GPIO is a simple MMIO operation.
>
> This patch provides definitions so a board can easily select the GPIO
> mode.
>
> NOTE: apparently, it's possible to run the geni in "GSI" mode.  In GSI
> the SPI port is allowed to be controlled by more than one user (like
> firmware and Linux) and also the port can operate sequences of
> operations in one go.  In GSI mode it _would_ be invalid to look at
> the chip select as a GPIO because that would prevent other users from
> using it.  In theory GSI mode would also avoid some overhead by
> allowing us to sequence the chip select better.  However, I'll argue
> GSI is not relevant for all boards (and certainly not any boards
> supported by mainline today).  Why?
> - Apparently to run a SPI chip in GSI mode you need to initialize it
>    (in the bootloader) with a different firmware and then it will
>    always run in GSI mode.  Since there is no support for GSI mode in
>    the current Linux driver, it must be that existing boards don't have
>    firmware that's doing that.  Note that the kernel device tree
>    describes hardware but also firmware, so it is legitimate to make
>    the assumption that we don't have GSI firmware in a given dts file.
> - Some boards with sc7180 have SPI connected to the Chrome OS EC or
>    security chip (Cr50).  The protocols for talking to cros_ec and cr50
>    are extremely complex.  Both drivers in Linux fully lock the bus
>    across several distinct SPI transfers.  While I am not an expert on
>    GSI mode it feels highly unlikely to me that we'd ever be able to
>    enable GSI mode for these devices.
>
>  From a testing perspective, running "flashrom -p ec -r /tmp/foo.bin"
> in a loop after this patch shows almost no reduction in time, but the
> number of interrupts per command goes from 32357 down to 30611 (about
> a 5% reduction).
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Stephen Boyd <swboyd@chromium.org>
> ---
>
> Reviewed-by: Akash Asthana <akashast@codeaurora.org>

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