linux-kernel - Re: [PATCH v2] i2c: rcar: add SMBus block read support

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Message-ID: <YkQ5LkUyicWFLlSJ@shikoro>
Date:   Wed, 30 Mar 2022 13:04:14 +0200
From:   Wolfram Sang <wsa+renesas@...g-engineering.com>
To:     "Gabbasov, Andrew" <Andrew_Gabbasov@...tor.com>
Cc:     "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "Surachari, Bhuvanesh" <Bhuvanesh_Surachari@...tor.com>
Subject: Re: [PATCH v2] i2c: rcar: add SMBus block read support

Hi Andrew,

thanks for your patience, I can finally work on this issue.

> > You could wire up two R-Car I2C instances, set up one as an I2C slave
> > handled by the I2C testunit and then use the other instance with
> > SMBUS_BLOCK_PROC_CALL which also needs RECV_LEN. Check
> > Documentation/i2c/slave-testunit-backend.rst for details.
> 
> You mean physical connection of two R-Car boards via I2C bus,
> or physical connection of I2C bus wires on the single board, right?

I have two instances on the same board wired.

> It looks like all the boards, that I have access to, do not have
> I2C bus wires exposed to some connectors, so both variants would
> require hardware re-wiring modification of the boards, which is
> not an option for me. Or do I understand you incorrectly and you
> mean something different?

Probably you understood correctly. Which boards do you have access to?
I use the EXIO connectors on a Salvator-X(S).

> Most of complexity in my patch is related to DMA transfers support,
> that I'm trying to retain for SMBus block data transfers too (for the rest
> of bytes after the first "length" byte). Your simple patch makes
> the driver perform all M_RECV_LEN transfers in PIO mode only (with no DMA at all),
> which is probably not quite good (it's a pity to loose existing HW capability,
> already supported by the driver).

I will have a look into RECV_LEN and DMA. I already started looking into
it but will need to dive in some more. Stay tuned, I hope to have the
next response ready this week.

> >  	/* If next received data is the _LAST_, go to new phase. */
> > -	if (priv->pos + 1 == msg->len) {
> > +	if (priv->pos + 1 == msg->len && !recv_len_init) {
> 
> If a message contains a single byte after the length byte,
> when we come here after processing the length (in the same function call),
> "pos" is 1, "len" is 2, and we indeed are going to process the last byte.
> However, "recv_len_init" is still "true", and we skip these corresponding
> register writes, which is probably incorrect.
> The flag in this case should be re-set back to "false" after length
> processing and "pos" moving, but I think the variant in my patch
> (leaving this "if" unchanged, but skipping it on the first pass with "goto")
> may be even simpler.

I also need to look into this but thank you already for the detailed
explanation!

> >  	u32 func = I2C_FUNC_I2C | I2C_FUNC_SLAVE |
> > -		   (I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK);
> > +		   (I2C_FUNC_SMBUS_EMUL_ALL & ~I2C_FUNC_SMBUS_QUICK);
> 
> This flags setting adds also I2C_FUNC_SMBUS_BLOCK_PROC_CALL flag,
> which is missed in my patch. My patch should probably be updated
> to include it too (if you'll agree to take my variant ;-) ).

Yes, the final version, whatever it will be, should use this new macro.

Until soon,

   Wolfram


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