linux-kernel - Re: [PATCH v2 05/11] spi: cadence-qspi: add FIFO depth detection quirk

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Date: Mon, 08 Apr 2024 16:38:56 +0200
From: Théo Lebrun <theo.lebrun@...tlin.com>
To: "Mark Brown" <broonie@...nel.org>
Cc: "Rob Herring" <robh+dt@...nel.org>, "Krzysztof Kozlowski"
 <krzysztof.kozlowski+dt@...aro.org>, "Conor Dooley" <conor+dt@...nel.org>,
 "Vaishnav Achath" <vaishnav.a@...com>, "Thomas Bogendoerfer"
 <tsbogend@...ha.franken.de>, "Rob Herring" <robh@...nel.org>,
 <linux-spi@...r.kernel.org>, <devicetree@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <linux-mips@...r.kernel.org>, "Vladimir
 Kondratiev" <vladimir.kondratiev@...ileye.com>, "Gregory CLEMENT"
 <gregory.clement@...tlin.com>, "Thomas Petazzoni"
 <thomas.petazzoni@...tlin.com>, "Tawfik Bayouk"
 <tawfik.bayouk@...ileye.com>
Subject: Re: [PATCH v2 05/11] spi: cadence-qspi: add FIFO depth detection
 quirk

Hello,

On Mon Apr 8, 2024 at 4:10 PM CEST, Mark Brown wrote:
> On Fri, Apr 05, 2024 at 05:02:15PM +0200, Théo Lebrun wrote:
>
> > Use hardware ability to read the FIFO depth thanks to
> > CQSPI_REG_SRAMPARTITION that is partially read-only. Keep current
> > behavior identical for existing compatibles.
>
> The behaviour is not identical here - we now unconditionally probe the
> FIFO depth on all hardware, the difference with the quirk is that we
> will ignore any DT property specifying the depth.

You are correct of course. Wording is incorrect. I wanted to highlight
that FIFO depth does not change for existing HW and still relies as
before on devicetree value.

> > -	if (of_property_read_u32(np, "cdns,fifo-depth", &cqspi->fifo_depth)) {
> > +	if (!(ddata && ddata->quirks & CQSPI_DETECT_FIFO_DEPTH) &&
> > +	    of_property_read_u32(np, "cdns,fifo-depth", &cqspi->fifo_depth)) {
> >  		dev_err(dev, "couldn't determine fifo-depth\n");
>
> It's not obvious from just the code that we do handle having a FIFO
> depth property and detection in the detection code, at least a comment
> would be good.

I see. Will add comment or rework code to make more straight forward, or
both.

> > +static void cqspi_controller_detect_fifo_depth(struct cqspi_st *cqspi)
> > +{
> > +	const struct cqspi_driver_platdata *ddata = cqspi->ddata;
> > +	struct device *dev = &cqspi->pdev->dev;
> > +	u32 reg, fifo_depth;
> > +
> > +	/*
> > +	 * Bits N-1:0 are writable while bits 31:N are read as zero, with 2^N
> > +	 * the FIFO depth.
> > +	 */
> > +	writel(U32_MAX, cqspi->iobase + CQSPI_REG_SRAMPARTITION);
> > +	reg = readl(cqspi->iobase + CQSPI_REG_SRAMPARTITION);
> > +	fifo_depth = reg + 1;
> > +
> > +	if (ddata && ddata->quirks & CQSPI_DETECT_FIFO_DEPTH) {
> > +		cqspi->fifo_depth = fifo_depth;
> > +		dev_dbg(dev, "using FIFO depth of %u\n", fifo_depth);
> > +	} else if (fifo_depth != cqspi->fifo_depth) {
> > +		dev_warn(dev, "detected FIFO depth (%u) different from config (%u)\n",
> > +			 fifo_depth, cqspi->fifo_depth);
> > +	}
>
> It's not obvious to me that we should ignore an explicitly specified
> property if the quirk is present

DT value isn't expected for compatibles with CQSPI_DETECT_FIFO_DEPTH
quirk, therefore we do not ignore a specified property. Bindings agree:
prop is false with EyeQ5 compatible.

> - if anything I'd more expect to see
> the new warning in that case, possibly with a higher severity if we're
> saying that the quirk means we're more confident that the data reported
> by the hardware is reliable.  I think what I'd expect is that we always
> use an explicitly specified depth (hopefully the user was specifying it
> for a reason?).

The goal was a simpler devicetree on Mobileye platform. This is why we
add this behavior flag. You prefer the property to be always present?
This is a only a nice-to-have, you tell me what you prefer.

I wasn't sure all HW behaved in the same way wrt read-only bits in
SRAMPARTITION, and I do not have access to other platforms exploiting
this driver. This is why I kept behavior reserved for EyeQ5-integrated
IP block.

> Pulling all the above together can we just drop the quirk and always do
> the detection, or leave the quirk as just controlling the severity with
> which we log any difference between detected and explicitly configured
> depths?

If we do not simplify devicetree, then I'd vote for dropping this patch
entirely. Adding code for detecting such an edge-case doesn't sound
useful. Especially since this kind of error should only occur to people
adding new hardware support; those probably do not need a nice
user-facing error message. What do you think?

Regards,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com