Message-ID: <YbdtLFSrwjYcz/zz@sirena.org.uk>
Date:   Mon, 13 Dec 2021 15:56:28 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Hector Martin <marcan@...can.st>
Cc:     Sven Peter <sven@...npeter.dev>, Rob Herring <robh+dt@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        linux-arm-kernel@...ts.infradead.org, linux-spi@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] spi: apple: Add driver for Apple SPI controller

On Mon, Dec 13, 2021 at 12:50:49PM +0900, Hector Martin wrote:
> On 13/12/2021 08.41, Mark Brown wrote:
> > On Sun, Dec 12, 2021 at 12:47:26PM +0900, Hector Martin wrote:

> > > @@ -0,0 +1,566 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/*
> > > + * Apple SoC SPI device driver
> > > + *

> > Please keep the entire comment a C++ one so things look more
> > intentional.

> I thought this pattern was pretty much the standard style.

It's common, especially given all the automated conversions, but ugly.

> > > +	/* We will want to poll if the time we need to wait is
> > > +	 * less than the context switching time.
> > > +	 * Let's call that threshold 5us. The operation will take:
> > > +	 *    bits_per_word * fifo_threshold / hz <= 5 * 10^-6
> > > +	 *    200000 * bits_per_word * fifo_threshold <= hz
> > > +	 */
> > > +	return 200000 * t->bits_per_word * APPLE_SPI_FIFO_DEPTH / 2 <= t->speed_hz;

> > Some brackets or an intermediate variable wouldn't hurt here, especially
> > given the line length.

> How about this?

> return (200000 * t->bits_per_word * APPLE_SPI_FIFO_DEPTH / 2) <= t->speed_hz;

That's better but it's still a very long line which is half the issue.

> > > +static const struct of_device_id apple_spi_of_match[] = {
> > > +	{ .compatible = "apple,spi", },
> > > +	{}
> > > +};
> > > +MODULE_DEVICE_TABLE(of, apple_spi_of_match);

> > This is an awfully generic compatible.  It's common to use the SoC name
> > for the IP compatibles when they're not otherwise identified?

> Apple like to keep blocks compatible across SoC generations - I think this
> one dates, at least to some extent, to the original iPhone or thereabouts.
> We do use per-SoC compatibles in the DTs in case we need to throw in per-SoC
> quirks in the future ("apple,t8103-spi", "apple,spi"), but for drivers like
> this we prefer to use generic compatibles as long as backwards compatibility
> doesn't break. If Apple do something totally incompatible (like they did
> with AIC2 in the latest chips), we'll bump to something like apple,spi2.
> This happens quite rarely, so it makes sense to just keep things generic
> except for these instances. That way old kernels will happily bind to the
> block in newer SoCs if it is compatible.

There's currently a bit of a fashion for people with very old SPI blocks
to make incompatible new versions recently, a lot of it seems to be
driven by things like flash engine support.  Sometimes these things end
up getting instantiated together as they have different purposes and the
incompatibilties make the IPs larger.

> If we had a detailed lineage of what SoCs used what blocks and when things
> changed we could try something else, like using the first SoC where the
> specific block version was introduced, but we don't... so I think it makes
> sense to just go with generic ones where we don't think things have changed
> much since the dark ages. FWIW, Apple calls this one spi-1,spimc and claims
> "spi-version = 1" and the driver has Samsung in the name... so the history
> of this block definitely goes back quite a ways :-)

Have you done a contrast and compare with the Samsung driver?  Given
both this and your comments above about this dating back to the original
iPhone...

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