RE: [PATCH v5 2/2] pinctrl: Add driver for Sunplus SP7021

From: "Wells Lu 呂芳騰" <wells.lu@sunplus.com>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Wells Lu <wellslutw@gmail.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	devicetree <devicetree@vger.kernel.org>,
	linux-arm Mailing List <linux-arm-kernel@lists.infradead.org>,
	"dvorkin@tibbo.com" <dvorkin@tibbo.com>
Subject: RE: [PATCH v5 2/2] pinctrl: Add driver for Sunplus SP7021
Date: Fri, 14 Jan 2022 11:36:31 +0000	[thread overview]
Message-ID: <cbfe10d1243f4d6bb37cf936f5926351@sphcmbx02.sunplus.com.tw> (raw)
In-Reply-To: <CAHp75VdTm5T=VGgrqfp+3bLAu4chosirJ2uBeoA5MxvV+FMg_Q@mail.gmail.com>

> ...
> 
> > > > > > > > > > +       bool "Sunplus SP7021 PinMux and GPIO driver"
> > > > > > > > >
> > > > > > > > > Why bool and not tristate?
> > > > > > > >
> > > > > > > > Pinctrl driver is selected by many drivers in SP7021 platform.
> > > > > > > > We never build it as a module, but build-in to kernel.
> > > > > > > > So we use "bool".
> > > > > > > >
> > > > > > > > Should we set it to tristate?
> > > > > > >
> > > > > > > You still haven't answered "why", so I can't tell you.
> > > > > >
> > > > > > I am puzzled because I think I have answered "why".
> > > > >
> > > > > Nope. :-)
> > > > >
> > > > > > Because Pinctrl driver is necessary for all SP7021-based platforms.
> > > > >
> > > > > "Why?" Why is it necessary (to be built-in)?
> > > >
> > > > Pinctrl is necessary to be built-in because drivers of
> > > > boot-device, like eMMC, SD card, NAND flash, and NOR flash drivers, need it.
> > > >
> > > > SP7021 supports booting from eMMC, SD card, NAND flash and NOR
> > > > flash devices. Their drivers need Pinctrl driver probes in advance.
> > >
> > > On x86 platforms, for example, booting from eMMC and SD card does
> > > not require a pin control driver to be built-in. Why is this requirement for SP
> platforms?
> 
> > Because all pins of boot-device multiplex with GPIO pins.
> >
> > SP7021 platforms support multi boot devices, including eMMC, SD card,
> > SPI-NOR flash, SPI-NAND flash. Each boot device has control-bit(s) to
> > enable it.
> >
> > For example #1, if control-bits of SPI-NOR flash is set 1, GPIO 83,
> > 84, 86, 87 are connected to SPI-NOR flash.
> > If control-bits of SPI-NOR flash is set 2, GPIO 76, 78, 79, 81 are
> > connected to SPI-NOR flash.
> > If control-bits of SPI-NOR flash is set 0, no pin is connected SPI-NOR
> > flash.
> >
> > For example #2, if control-bits of eMMC device is set 1, GPIO 72, 73,
> > 74, 75, 76, 77, 78, 79, 80, 81 are connected to eMMC device.
> > If control-bits of eMMC device is set 0, no pin is connected eMMC
> > device.
> >
> > For example #3, if control-bits of SDCard device is set 1, GPIO 65,
> > 66, 67, 68, 69, 70 are connected to SD Card device.
> > If control-bits of SDCard device is set 0, no pin is connected SDCard
> > device.
> >
> > Note that all pins multiplex with GPIO pins.
> >
> > Normally, only a boot-device is enabled. All other GPIO pins are
> > released for peripherals (controlled by fully-pinmux Or group pinmux)
> > or IO processor
> 
> Thanks for this detailed picture of the HW configuration.
> 
> > Pinctrl driver is responsible for setup the pins.
> > If Pinctrl fails to probe, boot-device also fails to probe.
> 
> But have you tried to compile it as a module and supply it in the initramfs?
> 
> What kernel does in this case it takes address of initramfs from memory (I believe
> you have bootloader that capable to setup kernel, initramfs, command line, and device
> tree blob and pass the control to the kernel) at some point, mounts it and probes the
> modules that are required to get your boot device ready.
> 
> --
> With Best Regards,
> Andy Shevchenko

Yes, I'll modify Kconfig and Makefile to support "compile as module".

Actually, SP7021 platforms also support booting from tftp and nfs server.
In these two cases, no boot device is enabled in Linux kernel (but 
boot device is enabled in U-Boot). Pinctrl driver can be a module.

Thanks,
Wells Lu