linux-kernel - Re: [PATCH v4 3/4] i2c: imx: use readb_relaxed and writeb

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Message-Id: <a65ddcab-4ba9-4dfe-93ec-352b31845eb4@app.fastmail.com>
Date: Wed, 02 Oct 2024 13:36:04 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "Stefan Eichenberger" <eichest@...il.com>
Cc: o.rempel@...gutronix.de,
 "Pengutronix Kernel Team" <kernel@...gutronix.de>,
 "Andi Shyti" <andi.shyti@...nel.org>, "Shawn Guo" <shawnguo@...nel.org>,
 "Sascha Hauer" <s.hauer@...gutronix.de>,
 "Fabio Estevam" <festevam@...il.com>, "Frank Li" <Frank.Li@....com>,
 linux-i2c@...r.kernel.org, imx@...ts.linux.dev,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 "Francesco Dolcini" <francesco.dolcini@...adex.com>,
 "Stefan Eichenberger" <stefan.eichenberger@...adex.com>
Subject: Re: [PATCH v4 3/4] i2c: imx: use readb_relaxed and writeb_relaxed

On Wed, Oct 2, 2024, at 13:08, Stefan Eichenberger wrote:
> On Wed, Oct 02, 2024 at 11:51:22AM +0000, Arnd Bergmann wrote:
>> On Wed, Oct 2, 2024, at 11:19, Stefan Eichenberger wrote:
>> > From: Stefan Eichenberger <stefan.eichenberger@...adex.com>
>> >
>> > Use the relaxed version of readb and writeb to reduce overhead. It is
>> > safe to use the relaxed version because we either do not rely on dma
>> > completion, or we use a dma callback to ensure that the dma transfer is
>> > complete before we continue.
>> 
>> I would still consider this a bug in general, you should
>> never default to the unsafe variants.
>> 
>> If there is a codepath that needs the barrierless version,
>> please add imx_i2c_write_reg_relaxed()/imx_i2c_read_reg_relaxed()
>> helpers that use those only in the places where it makes
>> a measurable difference, with a comment that explains
>> the usage.
>
> I added the patch because of the following dicussion:
> https://lore.kernel.org/linux-i2c/ZpVWXlR6j2i0ZtVQ@lizhi-Precision-Tower-5810/
>
> I can't determine if the relaxed version improves performance. The
> 'normal' version worked well for our use case too. Therefore, dropping
> the change would be acceptable for us. Another potential solution could
> be to use the relaxed version only inside the ISR. Would that be an
> acceptable solution? What is your impression, Frank Li
> <Frank.Li@....com>?

I'm pretty sure that Frank meant to use readb_relaxed()/writeb_relaxed()
inside of the FIFO access loop, not for everything else. This
makes a lot of sense, since the FIFO read in particular is
clearly performance sensitive and already serialized by the
implied control dependency.

If you can read multiple bytes, the best interface to use
would in fact be readsb() or possibly readsl() to read
four bytes with each access.

It appears that you did not implement the suggestion to
read the entire FIFO though, so you can probably just skip
the _relaxed() change entirely.

     Arnd