linux-kernel - Re: [PATCH v3] i2c: at91: support atomic write xfer

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Date:   Sun, 22 Mar 2020 17:30:13 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Wolfram Sang <wsa@...-dreams.de>
Cc:     Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Stefan Lengfeld <contact@...fanchrist.eu>,
        Marco Felsch <m.felsch@...gutronix.de>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] i2c: at91: support atomic write xfer

On Sun, Mar 22, 2020 at 03:30:04PM +0100, Wolfram Sang wrote:
> 
> > +	/* FIXME: only single write request supported to 7-bit addr */
> 
> Hmm, this is quite limited. Would it be very hard to support multiple
> messages? Or reads? 10 bits don't matter.

I don't expect this to be used for much more than a simple write to PMIC
to kill the power. So this patch is tailor made for exactly this purpose.
Though, if you would go for full support of atomic transfers, then
I would suggest to hack the non-atomic path to be usable in atomic mode
instead (some I2C drivers do just that, eg. i2c-tegra).

BTW, I found this comment in i2c-core.h:

 * We only allow atomic transfers for very late communication, e.g. to send
 * the powerdown command to a PMIC. Atomic transfers are a corner case and not
 * for generic use! 

I think this covers the idea.

> > +	if (!dev->pdata->has_alt_cmd)
> > +		return -EOPNOTSUPP;
> 
> We should handle this in probe(), I think:
> 
> 	if (dev->pdata->has_alt_cmd)
> 		at91_twi_algorithm.master_xfer_atomic = at91_twi_xfer_atomic;

This would mean writable ops structure - something I try hard to avoid.
We can use another copy of i2c_algorithm structure if needed, though.

Best Regards
Michał Mirosław