linux-kernel - Re: [PATCH 3/3] spi: apple: Add driver for Apple SPI controller

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Message-ID: <YbeI45NiYnhMzCSk@sirena.org.uk>
Date:   Mon, 13 Dec 2021 17:54:43 +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 Tue, Dec 14, 2021 at 02:10:26AM +0900, Hector Martin wrote:
> On 14/12/2021 00.56, Mark Brown wrote:
> > On Mon, Dec 13, 2021 at 12:50:49PM +0900, Hector Martin wrote:

> > > > 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.

> I think it's quite readable at this point (especially with the comment above
> explaining it anyway). Note that these days a lot of people consider lines
> up to 100 chars okay in the kernel, and checkpatch uses that limit. Do you
> have a specific change in mind?

The 100 characters is a "don't send silly checkpatch fixes" thing not a
target to aim for (see also the ternery operator stuff).  Like I said an
intermediate variable wouldn't hurt, for example for the FIFO trigger
level into a fifo_trigger variable.

> > 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.

> I think if they haven't changed it by now they probably won't; e.g. they
> tacked on DMA using a coprocessor instead of changing the block itself. I
> don't think Apple uses SPI for anything performance-critical. They don't
> even bother with QSPI for the NOR flash (which is mostly only used for
> bootloaders and variable storage).

This feels like tempting fate but I guess...

> > 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...

> You mean the *two* Samsung drivers? :-)

> It seems Samsung like to keep making up incompatible SPI blocks. This one
> shares a *few* bits in a *couple* registers with spi-s3c24xx driver, which
> point to a common lineage, but those registers aren't even at the same
> addresses. Not enough in common for it to make sense to try to use one
> driver for both (unlike with UART, where it was close enough to be added as
> a new Samsung UART variant, or I2C, where we could refactor the pasemi
> driver to add a platform backend alongside the existing PCI support and
> mostly use it as-is).

Their older SPI block has quite a few issues IIRC, I think DMA was the
big difference between the two but ICBW.

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