Re: [RFC PATCH V4 6/6] riscv: soc: Add Allwinner SoC kconfig option

[Heinrich Schuchardt <heinrich.schuchardt@canonical.com>]

On 9/13/21 10:45 AM, Maxime Ripard wrote:
> Hi,
> 
> On Sat, Sep 11, 2021 at 05:21:39PM +0800, guoren@kernel.org wrote:
>> From: Liu Shaohua <liush@allwinnertech.com>
>>
>> Add Allwinner kconfig option which selects SoC specific and common
>> drivers that is required for this SoC.
>>
>> Allwinner D1 uses custom PTE attributes to solve non-coherency SOC
>> interconnect issues for dma synchronization, so we set the default
>> value when SOC_SUNXI selected.
>>
>> Signed-off-by: Liu Shaohua <liush@allwinnertech.com>
>> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
>> Signed-off-by: Wei Fu <wefu@redhat.com>
>> Cc: Anup Patel <anup.patel@wdc.com>
>> Cc: Atish Patra <atish.patra@wdc.com>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Chen-Yu Tsai <wens@csie.org>
>> Cc: Drew Fustini <drew@beagleboard.org>
>> Cc: Maxime Ripard <maxime@cerno.tech>
>> Cc: Palmer Dabbelt <palmerdabbelt@google.com>
>> Cc: Wei Wu <lazyparser@gmail.com>
>> ---
>>   arch/riscv/Kconfig.socs      | 15 +++++++++++++++
>>   arch/riscv/configs/defconfig |  1 +
>>   2 files changed, 16 insertions(+)
>>
>> diff --git a/arch/riscv/Kconfig.socs b/arch/riscv/Kconfig.socs
>> index 30676ebb16eb..8721c000ef23 100644
>> --- a/arch/riscv/Kconfig.socs
>> +++ b/arch/riscv/Kconfig.socs
>> @@ -70,4 +70,19 @@ config SOC_CANAAN_K210_DTB_SOURCE
>>   
>>   endif
>>   
>> +config SOC_SUNXI
>> +	bool "Allwinner SoCs"
>> +	depends on MMU
>> +	select DWMAC_GENERIC
>> +	select ERRATA_THEAD
>> +	select RISCV_DMA_NONCOHERENT
>> +	select RISCV_ERRATA_ALTERNATIVE
>> +	select SERIAL_8250
>> +	select SERIAL_8250_CONSOLE
>> +	select SERIAL_8250_DW
>> +	select SIFIVE_PLIC
>> +	select STMMAC_ETH
>> +	help
>> +	  This enables support for Allwinner SoC platforms like the D1.
>> +
> 
> I'm not sure we should select the drivers there. We could very well
> imagine a board without UART, or even more so without ethernet.

The draft of the RISC-V platform specification is available here:
https://github.com/riscv/riscv-platform-specs/blob/main/riscv-platform-spec.adoc#uartserial-console

The specification requires in section "2.1.5.1. UART/Serial Console" 
that on platforms with a rich operating system (e.g. Linux) you have a 
serial console. Hence requiring 8250 support for the D1 CPU is justified.

In the riscv defconfig as of v5.14 we have:

CONFIG_SERIAL_8250=y
CONFIG_SERIAL_8250_CONSOLE=y
# CONFIG_SERIAL_8250_DW is not set
(Support for Synopsys DesignWare 8250 quirks)

CONFIG_SERIAL_8250_DW should be enabled (=y) in the defconfig.

As the specification requires a 16550 UART and marks 8250 as deprecated 
I expect that future Allwinner SoCs will move to 16550. Calling a 
Kconfig menu item "Allwinner SoCs" which includes all future Allwinner 
SoCs irritates me. How about CONFIG_SOC_SUNXI_D1 instead?

Why does the patch use 'depends on MMU' and does not 'select MMU'?

Best regards

Heinrich

> 
> These options should be in the defconfig.
> 
> Maxime
>