linux-kernel - Re: [PATCH 01/12] spi: dt-bindings: introduce the ``fifo-depth`` property

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Date: Fri, 9 Feb 2024 18:13:59 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Tudor Ambarus <tudor.ambarus@...aro.org>
Cc: broonie@...nel.org, robh@...nel.org, andi.shyti@...nel.org, 
	semen.protsenko@...aro.org, krzysztof.kozlowski@...aro.org, 
	alim.akhtar@...sung.com, linux-spi@...r.kernel.org, 
	linux-samsung-soc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, andre.draszik@...aro.org, 
	peter.griffin@...aro.org, kernel-team@...roid.com, willmcvicker@...gle.com, 
	conor+dt@...nel.org, devicetree@...r.kernel.org, arnd@...db.de, 
	Linux-Renesas <linux-renesas-soc@...r.kernel.org>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>
Subject: Re: [PATCH 01/12] spi: dt-bindings: introduce the ``fifo-depth`` property

Hi Tudor,

On Thu, Feb 8, 2024 at 2:51 PM Tudor Ambarus <tudor.ambarus@...aro.org> wrote:
> There are instances of the same IP that are configured by the integrator
> with different FIFO depths. Introduce the fifo-depth property to allow
> such nodes to specify their FIFO depth.
>
> We haven't seen SPI IPs with different FIFO depths for RX and TX, thus
> introduce a single property.

Ha...

Current documentation for the Clock-Synchronized Serial Interface with
FIFO (MSIOF) on e.g. R-Car Gen2 and later states:

    FIFO capacity: 32 bits × 64 stages for transmission and 32 bits ×
256 stages for reception

Initially (many years ago), there was some doubt about the validity
of these values (older variants on SH supported 64/64), hence
drivers/spi/spi-sh-msiof.c still has

    .tx_fifo_size = 64,
    .rx_fifo_size = 64,

Probably we should test and revisit this...

> --- a/Documentation/devicetree/bindings/spi/spi-controller.yaml
> +++ b/Documentation/devicetree/bindings/spi/spi-controller.yaml
> @@ -69,6 +69,11 @@ properties:
>           Should be generally avoided and be replaced by
>           spi-cs-high + ACTIVE_HIGH.
>
> +  fifo-depth:
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +    description:
> +      Size of the data FIFO in bytes.

I think it is prudent to consider the asymmetric case, too.
Whether that should be just two properties ("rx-fifo-depth" and
"tx-fifo-depth"), or also a third "fifo-depth", I defer to the DT
maintainers...

> +
>    num-cs:
>      $ref: /schemas/types.yaml#/definitions/uint32
>      description:

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds