linux-kernel - Re: [PATCH] i2c: axxia: Add I2C driver for AXM55xx

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Date:	Mon, 22 Sep 2014 13:04:55 +0200
From:	Wolfram Sang <wsa@...-dreams.de>
To:	Anders Berg <anders.berg@...gotech.com>
Cc:	devicetree <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-i2c@...r.kernel.org
Subject: Re: [PATCH] i2c: axxia: Add I2C driver for AXM55xx


> >> >> +     if (msg->len == 0 || msg->len > 255)
> >> >> +             return -EINVAL;
> >> >
> >> > Ouch, really? Maybe we should warn the user here.
> >>
> >> Yeah, the transfer length register limits the length to 255. I'll add
> >> a warning here.
> >
> > Please also add this information to the Kconfig description and
> > somewhere at the top of the source file. This is an important flaw which
> > people should easily find out about.
> >
> 
> You are referring to the "len <= 255" restriction being the flaw here,
> right? I'll add a note to Kconfig and the driver about that.

Yes, I meant that. I just remembered we should do something else:

Remove I2C_FUNC_I2C (because it cannot do endless transfers) from
functionality and simply use I2C_FUNC_SMBUS_I2C_BLOCK which does I2C
like transfers with the SMBus Limit of 32 bytes. It seems PMBus allows
for 255 byte which this HW could support, yet I don't recall we have
support for that size currently.

> The other part of the condition (msg->len == 0) should actually go
> away. The controller can do zero-length-data transfers. I'll fix that
> for next round.

Great!


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