linux-kernel - Re: [PATCH] i2c: rcar: add SMBus block read support

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Date:   Tue, 5 Oct 2021 15:31:35 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Andrew Gabbasov <andrew_gabbasov@...tor.com>
Cc:     Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Bhuvanesh Surachari <bhuvanesh_surachari@...tor.com>
Subject: Re: [PATCH] i2c: rcar: add SMBus block read support

Hi Andrew,

On Wed, Sep 22, 2021 at 6:14 PM Andrew Gabbasov
<andrew_gabbasov@...tor.com> wrote:
> The smbus block read is not currently supported for rcar i2c devices.
> This patchset adds the support to rcar i2c bus so that blocks of data
> can be read using SMbus block reads.(using i2c_smbus_read_block_data()
> function from the i2c-core-smbus.c).
>
> Inspired by commit 8e8782c71595 ("i2c: imx: add SMBus block read support")
>
> This patch (adapted) was tested with v4.14, but due to lack of real
> hardware with SMBus block read operations support, using "simulation",
> that is manual analysis of data, read from plain I2C devices with
> SMBus block read request.
>
> Signed-off-by: Bhuvanesh Surachari <bhuvanesh_surachari@...tor.com>
> Signed-off-by: Andrew Gabbasov <andrew_gabbasov@...tor.com>

Thanks for your patch!

> --- a/drivers/i2c/busses/i2c-rcar.c
> +++ b/drivers/i2c/busses/i2c-rcar.c
> @@ -429,9 +431,16 @@ static bool rcar_i2c_dma(struct rcar_i2c_priv *priv)
>                 /*
>                  * The last two bytes needs to be fetched using PIO in
>                  * order for the STOP phase to work.
> +                *
> +                * For SMBus block read the first byte was received using PIO.

So it might be easier to read, and more maintainable, to keep the
old assignments:

    buf = priv->msg->buf;
    len = priv->msg->len - 2;

and adjust them for SMBus afterwards:

    if (block_data) {
            /* For SMBus block read the first byte was received using PIO */
            buf++;
            len--;
    }

?

>                  */
> -               buf = priv->msg->buf;
> -               len = priv->msg->len - 2;
> +               if (block_data) {
> +                       buf = priv->msg->buf + 1;
> +                       len = priv->msg->len - 3;
> +               } else {
> +                       buf = priv->msg->buf;
> +                       len = priv->msg->len - 2;
> +               }
>         } else {
>                 /*
>                  * First byte in message was sent using PIO.

And below we have another case handling buf and len :-(

So perhaps:

    buf = priv->msg->buf;
    len = priv->msg->len;

    if (read) {
            /*
             * The last two bytes needs to be fetched using PIO in
             * order for the STOP phase to work.
             */
            len -= 2;
    }
    if (!read || block_data) {
            /* First byte in message was sent using PIO *
            buf++;
            len--;
    }

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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