From:   Alexander Stein <alexander.stein@ew.tq-group.com>
To:     Tim Harvey <tharvey@gateworks.com>
Cc:     Rob Herring <robh+dt@kernel.org>, Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        Fabio Estevam <festevam@gmail.com>,
        NXP Linux Team <linux-imx@nxp.com>,
        Device Tree Mailing List <devicetree@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Linux ARM Mailing List <linux-arm-kernel@lists.infradead.org>
Subject: Re: (EXT) Re: (EXT) Re: [PATCH] arm64: dts: imx8mm-venice-gw73xx-0x: add dt overlays for serial modes
Date:   Wed, 12 Jan 2022 07:58:00 +0100
Message-ID: <6686721.lOV4Wx5bFT@steina-w>
Organization: TQ-Systems GmbH
In-Reply-To: <CAJ+vNU2PxF=9VwMv4f8N5W5Gs2Ynxdn9jHTSkWH7zd3Fo5hBiw@mail.gmail.com>
References: <20211214213630.14819-1-tharvey@gateworks.com> <2226437.ElGaqSPkdT@steina-w> <CAJ+vNU2PxF=9VwMv4f8N5W5Gs2Ynxdn9jHTSkWH7zd3Fo5hBiw@mail.gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 7Bit
Content-Type: text/plain; charset="us-ascii"
Precedence: bulk

Am Dienstag, 11. Januar 2022, 18:53:29 CET schrieb Tim Harvey:
> On Mon, Jan 10, 2022 at 11:20 PM Alexander Stein
> 
> <alexander.stein@ew.tq-group.com> wrote:
> > Am Dienstag, 11. Januar 2022, 01:00:21 CET schrieb Tim Harvey:
> > > [SNIP]
> > > 
> > > > diff --git a/arch/arm64/boot/dts/freescale/Makefile
> > > > b/arch/arm64/boot/dts/freescale/Makefile index
> > > > a14a6173b765..5ec8d59347b6
> > > > 100644
> > > > --- a/arch/arm64/boot/dts/freescale/Makefile
> > > > +++ b/arch/arm64/boot/dts/freescale/Makefile
> > > > @@ -44,6 +44,9 @@ dtb-$(CONFIG_ARCH_MXC) +=
> > > > imx8mm-var-som-symphony.dtb
> > > > 
> > > >  dtb-$(CONFIG_ARCH_MXC) += imx8mm-venice-gw71xx-0x.dtb
> > > >  dtb-$(CONFIG_ARCH_MXC) += imx8mm-venice-gw72xx-0x.dtb
> > > >  dtb-$(CONFIG_ARCH_MXC) += imx8mm-venice-gw73xx-0x.dtb
> > > > 
> > > > +dtb-$(CONFIG_ARCH_MXC) += imx8mm-venice-gw73xx-0x-rs232-rts.dtbo
> > > > +dtb-$(CONFIG_ARCH_MXC) += imx8mm-venice-gw73xx-0x-rs422.dtbo
> > > > +dtb-$(CONFIG_ARCH_MXC) += imx8mm-venice-gw73xx-0x-rs485.dtbo
> > > > 
> > > >  dtb-$(CONFIG_ARCH_MXC) += imx8mm-venice-gw7901.dtb
> > > >  dtb-$(CONFIG_ARCH_MXC) += imx8mm-venice-gw7902.dtb
> > > >  dtb-$(CONFIG_ARCH_MXC) += imx8mn-beacon-kit.dtb
> > > 
> > > [SNIP]
> > > I'm mostly interested to see if my approach to dt fragments here and
> > > the naming of the files makes sense to others.
> > > 
> > > This patch causes the kernel to build dtbo files for:
> > > arch/arm64/boot/dts/freescale/imx8mm-venice-gw73xx-0x-rs232-rts.dtbo
> > > arch/arm64/boot/dts/freescale/imx8mm-venice-gw73xx-0x-rs422.dtbo
> > > arch/arm64/boot/dts/freescale/imx8mm-venice-gw73xx-0x-rs485.dtbo
> > > 
> > > The intention is that these files are used by boot firmware (U-Boot)
> > > to adjust the dtb before passing it to the kernel.
> > 
> > Hi Tim,
> > 
> > do these dtbo actually work? I'm wondering because I was trying to
> > useoverlays myself and noticed that the had to be compiled with -@ for
> > u-boot to be able
> > to apply them. Apparently there are 2 possibilities:
> Alexander,
> 
> Yes, they work, but I do manually set DTC_FLAGS=-@ when building
> kernel dtbs to make them work.

I see, I expected something like this. That's why I responded to you.

> > * Set "DTC_FLAGS_[dtb] := -@" yourself
> > See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
> > commit/?id=e426d63e752bdbe7d5ba2d872319dde9ab844a07
> > 
> > * Use dedicated overlay target
> > See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
> > commit/?id=15d16d6dadf6947ac7f9a686c615995c5a426ce2
> > 
> > You use neither of them. IIRC just naming the target file .dtbo will not
> > apply symbols (-Q) during dtc call. Can you verify using 'V=1'
> > Also I'm wondering which way is the best to go.
> 
> I wasn't aware there was a way to do this via Makefiles. It seems that
> perhaps Rob's approach with 'kbuild: Add generic rule to apply
> fdtoverlay' is a way to avoid having to add them all manually in the
> first approach? I must admit I'm not sure how to use that.

I tried using this myself for my custom board. I feel it is a bit cumbersome 
to get it right.
See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/
arch/arm64/boot/dts/xilinx/Makefile for an example.

Essentially you define your .dtb as before and add another target (e.g. sm-
k26-revA-sck-kv-g-revA-dtbs) where you add your .dtbo _after_ the original 
.dtb. This target needs to be added to 'dtb-y' as before.

I suspect this way is needed to check the .dtbo against the base .dtb if it 
actually matches.

HTH
Alexander