Re: [PATCH 2/2] mfd: Add Baikal-T1 Boot Controller driver

From: Sergey Semin
Date: Fri Mar 27 2020 - 06:46:00 EST

On Fri, Mar 27, 2020 at 09:01:39AM +0000, Lee Jones wrote:
> On Thu, 26 Mar 2020, Sergey Semin wrote:
>
> > Michael, Richard, Vignesh and MTD mailing list are Cc'ed to have their
> > comments on the issue.
> >
> > My answers on the previous email are below.
> >
> > On Thu, Mar 26, 2020 at 09:13:13AM +0000, Lee Jones wrote:
> > > On Wed, 25 Mar 2020, Sergey Semin wrote:
> > >
> > > > Hello Lee,
> > > >
> > > > On Wed, Mar 25, 2020 at 10:09:40AM +0000, Lee Jones wrote:
> > > > > On Fri, 06 Mar 2020, Sergey.Semin@xxxxxxxxxxxxxxxxxxxx wrote:
> > > > >
> > > > > > From: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
> > > > > >
> > > > > > Baikal-T1 Boot Controller is an IP block embedded into the SoC and
> > > > > > responsible for the chip proper pre-initialization and further
> > > > > > booting up from some memory device. From the Linux kernel point of view
> > > > > > it's just a multi-functional device, which exports three physically mapped
> > > > > > ROMs and a single SPI controller.
> > > > > >
> > > > > > Primarily the ROMs are intended to be used for transparent access of
> > > > > > the memory devices with system bootup code. First ROM device is an
> > > > > > embedded into the SoC firmware, which is supposed to be used just for
> > > > > > the chip debug and tests. Second ROM device provides a MMIO-based
> > > > > > access to an external SPI flash. Third ROM mirrors either the Internal
> > > > > > or SPI ROM region, depending on the state of the external BOOTCFG_{0,1}
> > > > > > chip pins selecting the system boot device.
> > > > > >
> > > > > > External SPI flash can be also accessed by the DW APB SSI SPI controller
> > > > > > embedded into the Baikal-T1 Boot Controller. In this case the memory mapped
> > > > > > SPI flash region shouldn't be accessed.
> > > > > >
> > > > > > Taking into account all the peculiarities described above, we created
> > > > > > an MFD-based driver for the Baikal-T1 controller. Aside from ordinary
> > > > > > OF-based sub-device registration it also provides a simple API to
> > > > > > serialize an access to the external SPI flash from either the MMIO-based
> > > > > > SPI interface or embedded SPI controller.
> > > > >
> > > > > Not sure why this is being classified as an MFD.
> > > > >
> > > > > This is clearly 'just' a memory device.
> > > > >
> > > >
> > > > Hm, I see this as a normal MFD device. The Boot controller provides a
> > > > set of physically mapped ROMs and a DW APB SSI-based embedded SPI
> > > > controller. Yes, the SPI controller is normally utilized to access an external
> > > > flash device, and at boot stage it is used for it. But still it's a SPI
> > > > controller which driver belongs to the kernel SPI subsystem. Moreover
> > > > nothing prevents a platform designer from using it together with custom
> > > > GPIO-based chip-selects to have additional devices on the SPI bus.
> > > >
> > > > As I said the problem is that an SPI flash might be accessed either with
> > > > use of a physically mapped ROM or via the normal DW APB SPI controller.
> > > > These two interfaces can't be used simultaneously, because the ROM is
> > > > just an rtl state-machine, which is built to translate MMIO operations
> > > > through the SPI controller registers to an external SPI-nor at CS0 of
> > > > the interface. That's why first I need to make sure the interface is locked,
> > > > then being enabled, then the corresponding driver can use it, then get
> > > > to unlock. That's the point of having the __bt1_bc_spi_lock() and
> > > > bt1_bc_spi_unlock() methods exported from the driver.
> > > >
> > > > I've got two drivers for MTD ROM and SPI controller based on that
> > > > methods. But I haven't submitted them yet, because they belong to two
> > > > different subsystems and I need to have this one being accepted first.
> > >
> > > This is not a totally unique device/situation. I've seen (and NACKed)
> > > this type of device before. You need to explain this to the MTD
> > > (SPI-NOR?) maintainers. They should be in a good position to provide
> > > guidance.
> > >
> >
> > Sorry, I don't really understand your justification. The boot controller
> > exports two types of devices: physically mapped ROMs and an DW APB
> > SSI-based SPI controller. Aside from being able to access an externally
> > attached SPI flash the SPI controller has normal SPI interface which in
> > general can be used to access any other SPI-slave. The complexity of
> > the case is that one of physically mapped ROM RTL uses the DW APB SSI
> > controller to access an external SPI flash, which when done makes the
> > DW APB SSI registers unusable on direct way. That's why I implemented the
> > boot controller as an MFD. An alternation caused by this peculiarity
> > will be submitted to drivers/mtd/maps/physmap-{core.c,baikal-t1-rom.c}
> > later after this change is reviewed and accepted. Similar situation
> > concerns a driver of DW APB SSI-based SPI controller. So according to
> > the current design the boot controller with it' sub-devices will be
> > declared in dts as follows:
> >
> > boot: boot@1f040000 {
> > compatible = "be,bt1-boot-ctl";
> > reg = <0x1f040000 0x100>;
> > #address-cells = <1>;
> > #size-cells = <1>;
> > ranges;
>
> What control does this device offer in those 0x100 registers?
>

The main functionality of these registers is to switch the DW APB SSI
controller registers utilization: either make them being normally mapped
to the spi0:reg MMIO space (see spi0 node below) or set them being utilized
by the SPI ROM RTL, which is mapped to the spi_rom:reg ROM space. So in
first case the SPI-interface is handled by a generic SPI-controller,
while in the second one - it's purely subject of the SPI-flash access.

> > clocks = <&ccu_sys CCU_SYS_APB_CLK>;
> > clock-names = "pclk";
> >
> > int_rom: rom@1bfc0000 {
> > compatible = "be,bt1-int-rom", "mtd-rom";
> > reg = <0x1bfc0000 0x10000>;
> > ...
> > };
> >

BTW Normally I add an SPI-flash subnode to the SPI-controller, which
will refer to the same SPI-flash as spi_rom:

> > spi_rom: rom@1c000000 {
> > compatible = "be,bt1-ssi-rom", "mtd-rom";
> > reg = <0x1c000000 0x1000000>;
> > ...
> > };
> >
> > spi0: spi@1f040100 {
> > compatible = "be,bt1-boot-ssi";
> > reg = <0x1f040100 0xf00>;
> > #address-cells = <1>;
> > #size-cells = <0>;
> >
> > clocks = <&ccu_sys CCU_SYS_SPI_CLK>;
> > clock-names = "ref";
> >
spi_flash: flash@0 {
compatible = "jedec,spi-nor";
#address-cells = <1>;
#size-cells = <1>;
reg = <0>;

spi-max-frequency = <25000000>;
};
> > };
> >

Now both spi_rom and spi_flash devices can be used to access the same
memory device. But in first case it would be the read-only access in
4-bytes chunks, while in the second case - read-write in up to the page
chunks. But in addition to the spi_flash, I could place there any other
SPI-slave device, like SPI-UART controller or SPI-SD selected by a GPIO
chip-select line, and normally this should work.

NOTE also that by adding the SPI-flash sub-node of spi0 in addition to
the spi_rom I totally follow the device tree requirement to reflect the
real hardware devices structure. So from DT point of view such
construction is totally allowable and even required.

> > boot_rom: rom@1fc00000 {
> > compatible = "be,bt1-boot-rom", "mtd-rom";
> > reg = <0x1fc00000 0x400000>;
> > ...
> > };
> > };
> >
> > I dare to assume, that by saying: "Despite including the MFD API, I don't
> > see it being used at all." you meant that since the driver doesn't
> > redistribute any resource by declaring mfd_cell'es, doesn't
> > register mfd-based sub-devices, and primary use-case of the boot
> > controller is to access flash-devices, it should be just moved to the MTD
> > subsystem. I don't think it would be better than to have a common part
> > defined here in MFD while ROM-specific part - in MTD, and SPI-specific - in
> > the SPI subsystems. I would consider Baikal-T1 Boot Controller being similar
> > to drivers/mfd/qcom-spmi-pmic.c, drivers/mfd/atmel-flexcom.c, etc, but
> > instead of having GPIO, RTC, UART, i2c, etc sub-devices (which are also
> > fully defied in dts), it exports MMIO-based ROMs and SPI-controller.
>
> Are the ROMs all controlled via SPI?
>

No. int_rom is embedded into the Boot Controller firmware. It has
nothing concerning SPI. boot_rom is just a mirror of either spi_rom or int_rom
depending on the system bootup mode (selected by external pins settings).
So if SPI BOOT mode is enabled at the system startup, then boot_rom =
spi_rom (at first 4MB range), if ROM BOOT mode is enabled, then boot_rom = int_rom.

> > Lee, explain me please what is the difference between these MFDs and
> > Baikal-T1 Boot Controller, that makes one simple MFDs suitable for the
> > MFD subsystem, while another isn't?
>
> Complexity.
>
> [NB: Please Don't assume that just because I accepted a driver into
> MFD 6 years ago, that it would be accepted again today.]
>

Does this mean that accepting Qcom SPMI PMIC and Atmel flexcom drivers
to the MFD would be a mistake from current MFD point of view? What
changed since then?

> To me this looks like an MTD device which is controlled via SPI.
>

If it was like this I wouldn't have had it submitted to the MFD subsystem.
Boot Controller has multiple sub-blocks embedded not only ROMs. By
saying so I refer to the DW APB SSI-based SPI-controller and it'
sub-nodes. See the next comment for details.

> The way I'm reading this currently, it might serve well to make the
> MTD portion(s) children of the SPI controller. I still do not see a
> compelling reason to warrant adding a superfluous MFD driver if at all
> avoidable.
>

Here what the Boot Controller is from hardware point of view. It's an RTL,
which includes the next functionality/subblocks:
1) Basic system initialization at the chip startup stage, when it's
powered on (like resetting the subblocks, strapping the boot mode, etc).
2) Internal ROM with a default firmware, which is embedded into the chip
and is run at the system startup when the corresponding boot mode is enabled.
Firmware is mapped to the int_rom:reg registers.
3) DW APB SSI-based SPI-controller, though with very limited resources
(no IRQ, no DMA, just 8 bytes FIFO). So it's just a normal SPI-controller.
4) SPI ROM RTL, which allows to access an externally attached 16MB SPI-nor at
SPI chip-select #0 via the spi_rom:reg memory space. When the
corresponding flag in the Boot Controller register is set the DW APB SSI
registers are acquired by this RTL and unattached from the spi0:reg
space to access the external SPI flash.
5) Boot ROM memory space, which is just a mirror of either SPI ROM or
Internal ROM depending on the boot mode pins state strapped at the chip
startup stage.

NOTE SPI ROM RTL (spi_rom) allows just READ-only access to the SPI-flash
and makes it in 4-bytes data chunks at a time which isn't very efficient.
While by means of the direct DW APB SSI controller usage (via spi0) the
SPI-flash can be read in much bigger chunks and, which is crucial, can be
written. Moreover if GPIO chip-select is available the SPI-controller can be
attached to any other SPI-slave (not only SPI-flash).

So, yes, Baikal-T1 Boot Controller is less complex than Qcom SPMI or
Atmel Flexcom, but as I see it, the controller still has multi-function
nature: ROMs + a specific SPI-controller.

If you are still in doubts whether the Boot Controller should be
identified as MFD and don't agree to have the driver here, then where
do you think it is supposed to go? To an SPI subsystem? No, because
it has embedded ROMs. To MTD? No, It has the SPI-controller. I thought,
that MFD is for any device, which includes blocks of different
functionalities (I2C, GPIO, SPI, ROMs, RTC, UART, etc). Which as I see it
Baikal-T1 Boot Controller is.

-Sergey

> > Miquel, Richard, Vignesh and everyone from MTD, who can help could you
> > join this discussion and clarify whether the Baikal-T1 Boot Controller
> > driver is supposed to be moved to the MTD subsystem? If so, then what is
> > the better place to put it within the drivers/mtd/ directory tree?
> >
> > > > Recently I've sent an RFC regarding a different question, but it
> > > > concerns the Baikal-T1 system controller and the boot controller as being part
> > > > of it:
> > > >
> > > > https://lkml.org/lkml/2020/3/22/393
> > >
>
> --
> Lee Jones [æçæ]
> Linaro Services Technical Lead
> Linaro.org â Open source software for ARM SoCs
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