linux-kernel - Re: [PATCH 571/606] serial: sc16is7xx: Convert to i2c's .probe

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Message-ID: <20221123080912.lbmfgnco67psdu27@pengutronix.de>
Date:   Wed, 23 Nov 2022 09:09:12 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     kernel@...gutronix.de,
        Uwe Kleine-König <uwe@...ine-koenig.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Wolfram Sang <wsa@...nel.org>,
        Angel Iglesias <ang.iglesiasg@...il.com>,
        linux-i2c@...r.kernel.org, linux-serial@...r.kernel.org,
        Grant Likely <grant.likely@...aro.org>,
        Lee Jones <lee.jones@...aro.org>
Subject: Re: [PATCH 571/606] serial: sc16is7xx: Convert to i2c's .probe_new()

Hello Jiri,

On Wed, Nov 23, 2022 at 07:36:52AM +0100, Jiri Slaby wrote:
> On 21. 11. 22, 8:07, Uwe Kleine-König wrote:
> > On Mon, Nov 21, 2022 at 07:03:41AM +0100, Jiri Slaby wrote:
> > > On 18. 11. 22, 23:45, Uwe Kleine-König wrote:
> > > > From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> > > > 
> > > > .probe_new() doesn't get the i2c_device_id * parameter, so determine
> > > > that explicitly in the probe function.
> > > 
> > > I wonder why -- is this a new approach to probe functions? Or is only i2c
> > > affected? And why? Could you point to the commit introducing and describing
> > > the change in the i2c core?
> > 
> > I didn't sent the cover letter to all recipents of the individual
> > patches, so flow of information is a bit rough. Sorry about that.
> > 
> > You can find it at
> > https://lore.kernel.org/lkml/20221118224540.619276-1-uwe@kleine-koenig.org/,
> > it should answer your question.
> 
> Yes, I looked up that beforehand, but was no more clever after reading it.
> 
> > The short version is: The i2c framework does a more or less expensive
> > lookup for each call to .probe() to provide the id parameter. A relevant
> > part of the drivers however doesn't use this parameter, so the idea is
> > to let the drivers who actually need it, determine it themselves.
> > 
> > Statistics for the current state of this series in my tree:
> > Among the 602 converted drivers, 404 don't make use of the parameter.
> 
> So doesn't it make sense to provide both probe with no id and "probe_id"
> then? 200 is quite a few (a third to be precise).

Having the probe callback with the id parameter is only temporary. As
soon as all drivers are converted, the variant with the id parameter
will go away.

> BTW is this a performance issue? I.e. does it slow down the boot?

I don't know the start motivation for Lee (who triggered the conversion
in b8a1a4cd5a98 ("i2c: Provide a temporary .probe_new() call-back
type")).
Looking at the git history, he created 1e98dcd77970 ("mfd: 88pm860x:
Move over to new I2C device .probe() call") converting a driver that
doesn't benefit immensely. The lookup is more expensive for drivers with
big .id_table, the converted driver has only one entry.

I think in the end is a mixture between:

 - A big part of the drivers doesn't benefit from the lookup.
 - For most other busses the probe function only gets a device parameter
   and no id (spi, platform, i3c). There are counter examples though:
   amba, usb. Didn't check further.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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