linux-kernel - Re: [PATCH v2 23/28] spi: s3c64xx: retrieve the FIFO size from the device tree

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Date: Fri, 26 Jan 2024 14:20:15 -0600
From: Sam Protsenko <semen.protsenko@...aro.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Mark Brown <broonie@...nel.org>, Tudor Ambarus <tudor.ambarus@...aro.org>, 
	Andi Shyti <andi.shyti@...nel.org>, Rob Herring <robh+dt@...nel.org>, 
	krzysztof.kozlowski+dt@...aro.org, Conor Dooley <conor+dt@...nel.org>, 
	Alim Akhtar <alim.akhtar@...sung.com>, linux-spi@...r.kernel.org, 
	linux-samsung-soc@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	Linux-Arch <linux-arch@...r.kernel.org>, André Draszik <andre.draszik@...aro.org>, 
	Peter Griffin <peter.griffin@...aro.org>, kernel-team@...roid.com, 
	William McVicker <willmcvicker@...gle.com>
Subject: Re: [PATCH v2 23/28] spi: s3c64xx: retrieve the FIFO size from the
 device tree

On Fri, Jan 26, 2024 at 2:17 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Fri, Jan 26, 2024, at 20:23, Sam Protsenko wrote:
> > On Thu, Jan 25, 2024 at 11:33 AM Mark Brown <broonie@...nel.org> wrote:
> >>
> >> On Thu, Jan 25, 2024 at 02:50:01PM +0000, Tudor Ambarus wrote:
> >>
> >> > Allow SoCs that have multiple instances of the SPI IP with different
> >> > FIFO sizes to specify their FIFO size via the "samsung,spi-fifosize"
> >> > device tree property. With this we can break the dependency between the
> >> > SPI alias, the fifo_lvl_mask and the FIFO size.
> >>
> >> OK, so we do actually have SoCs with multiple instances of the IP with
> >> different FIFO depths (and who knows what else other differences)?
> >
> > I think that's why we can see .fifo_lvl_mask[] with different values
> > for different IP instances. For example, ExynosAutoV9 has this (in
> > upstream driver, yes):
> >
> >     .fifo_lvl_mask = { 0x1ff, 0x1ff, 0x7f, 0x7f, 0x7f, 0x7f, 0x1ff,
> > 0x7f, 0x7f, 0x7f, 0x7f, 0x7f},
> >
>
> That sounds like the same bug as in the serial port driver,
> by assuming that the alias values in the devicetree have
> a particular meaning in identifying instances. This immediately
> breaks when there is a dtb file that does not use the same
> alias values, e.g. because it only needs some of the SPI
> ports.
>

Exactly. I guess that's exactly what Tudor mentioned in his commit
message, and he's trying to fix that very problem by relying on
corresponding dts property (in his patch series) rather than on
fifo_lvl_mask.

>       Arnd