Re: [PATCH V2 22/22] MIPS: Add multiplatform BMIPS target

From: Kevin Cernekee
Date: Sun Nov 16 2014 - 17:13:14 EST

On Sun, Nov 16, 2014 at 1:24 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Saturday 15 November 2014 16:17:46 Kevin Cernekee wrote:
>> + bool "Broadcom BCM33xx/BCM63xx/BCM7xxx multiplatform kernel"
>> + select BOOT_RAW
>> + select NO_EXCEPT_FILL
>> + select USE_OF
>> + select BUILTIN_DTB
>> + select FW_CFE
>> + select CEVT_R4K
>> + select CSRC_R4K
>> + select SYNC_R4K
>> + select COMMON_CLK
>> + select BCM7038_L1_IRQ
>> + select BCM7120_L2_IRQ
>> + select BRCMSTB_L2_IRQ
>> + select IRQ_CPU
>> + select SYS_HAS_CPU_BMIPS3300
>> + select SYS_HAS_CPU_BMIPS4350
>> + select SYS_HAS_CPU_BMIPS4380
>> + select SYS_HAS_CPU_BMIPS5000
>> + select SWAP_IO_SPACE
> You mentioned in another thread that all MMIO is byteswapped based on the
> jumper setting. Should the USB options have an 'if CPU_BIG_ENDIAN'
> behind them? I think it will still work in the current way, but when you
> know that you have little-endian registers, it is more efficient
> not to set these.

Right, it works OK this way because the USB_*_BIG_ENDIAN_* options
just allow the USB drivers to accept the "big-endian" DT property.
They don't actually force any endian swaps on their own.

I can add the extra condition if warranted. On an LE build it saves
about 19kB = 0.3% in .text:

$ size vmlinux vmlinux.orig
text data bss dec hex filename
5945044 6227964 263936 12436944 bdc5d0 vmlinux
5963924 6227964 263936 12455824 be0f90 vmlinux.orig

("data" includes an initramfs so it's huge, but removing it doesn't
impact the "text" column.)

At some point I'd also like to get the of_device_is_big_endian() patch
merged, and then use it to harmonize the way endian swapping works
across serial8250, libahci, and maybe USB. Depending on the
size/complexity impact and the level of passion surrounding the topic,
this might also involve getting rid of the compile-time optimizations,
or at least simplifying them down to a single option instead of 6.

>> + help
>> + Build a multiplatform DT-based kernel image that boots on select
>> + BCM33xx cable modem chips, BCM63xx DSL chips, and BCM7xxx set-top
>> + must be set appropriately for your board, and the dts/dtsi files may
>> + require changes based on the system endianness.
> Can you clarify the meaning of "multiplatform" here? I suspect you
> mean that this is one platform that includes multiple SoCs that
> are all made by the same manufacturer and based on the same CPU
> core family, but it is still incompatible with other Vendor's SoCs
> and with Broadcom MIPS SoCs that are based on 74K or other cores, right?


> It's probably not a wrong description here, but for anybody reading this
> who also works on ARM, it seems rather confusing because there,
> "multiplatform" implies that the particular SoC can be built into a
> generic kernel image that supports SoCs from any vendor whose platform
> is also marked as "multiplatform", as long as the CPU architecture level
> (v4/v5, or v6/v7, or v8) is the same.

The BMIPS multiplatform kernel is intended to support any SoC based on
a 65nm/40nm/28nm BMIPS CPU. Strictly speaking, "BMIPS" isn't an
architecture level defined by imgtec, nor is it something that other
silicon vendors can currently offer. But the BMIPS CPUs do have their
own unique CP0 registers, DSP instruction set, errata, and ways of
handling SMP / cache maintenance / performance counters.

Outside of the CPU, the BCM63xx/BCM33xx/BCM7xxx register maps and
peripherals look pretty different, and the arch/mips/bmips code makes
almost zero assumptions about the rest of the chip if a DTB is passed
in from the bootloader. In this sense you can see the parallels to

Prior to this work, these product lines have never been able to share
a common kernel image.
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