Re: [RFC PATCH 3/5] i2c: core: add function to request an alias

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Wolfram Sang <wsa@the-dreams.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Luca Ceresoli <luca@lucaceresoli.net>,
	Wolfram Sang <wsa+renesas@sang-engineering.com>,
	Linux I2C <linux-i2c@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	Kieran Bingham <kieran@ksquared.org.uk>,
	Jacopo Mondi <jacopo@jmondi.org>,
	Vladimir Zapolskiy <vz@mleia.com>
Subject: Re: [RFC PATCH 3/5] i2c: core: add function to request an alias
Date: Wed, 8 Jan 2020 14:29:07 +0100	[thread overview]
Message-ID: <CAMuHMdX4mCw3pSGEzJUk9PRtnTwu-_R7TdxdDhJgzPAdbVKi1Q@mail.gmail.com> (raw)
In-Reply-To: <20200108131929.GA834@kunai>

Hi Wolfram,

On Wed, Jan 8, 2020 at 2:20 PM Wolfram Sang <wsa@the-dreams.de> wrote:
> > > > As I said to Laurent, too, I think the risk that a bus is not fully
> > > > described is higher than a device which does not respond to a read_byte.
> > > > In both cases, we would wrongly use an address in use.
> >
> > I don't fully agree with this, I think we shouldn't impose a penalty on
> > every user because some device trees don't fully describe the hardware.
>
> I haven't decided yet. However, my general preference is that for a
> generic OS like Linux, saftey comes first, then performance. If you have
> a fully described DT, then the overhead will be 1 read_byte transaction
> per requested alias at probe time. We could talk about using quick_read
> to half the overhead. You could even patch it away, if it is too much
> for $customer.
>
> > I think we should, at the very least, skip the probe and rely on DT if
> > DT explicitly states that all used addresses are listed. We discussed a
> > property to report addresses used by devices not described in DT, if
> > that property is listed I would prefer trusting DT.
>
> Yeah, we discussed this property and I have no intentions of dropping
> it. I haven't though of including it into this series, but it probably
> makes sense. We don't have to define much anyhow, just state what
> already exists, I guess.
>
> From Documentation/devicetree/bindings/i2c/i2c-ocores.txt:
>
>         dummy@60 {
>                 compatible = "dummy";
>                 reg = <0x60>;
>         };
>
> I think "dummy" is generic enough to be described in i2c.txt.

Dummy-the-node or dummy-the-compatible value?
Probably dummy nodes should have no compatible value at all?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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