Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support

Damien Le Moal
2021-05-19T08:25:55Z

On 2021/05/19 16:16, Anup Patel wrote:
> On Wed, May 19, 2021 at 12:24 PM Drew Fustini <drew@beagleboard.org> wrote:
>>
>> On Wed, May 19, 2021 at 08:06:17AM +0200, Christoph Hellwig wrote:
>>> On Wed, May 19, 2021 at 02:05:00PM +0800, Guo Ren wrote:
>>>> Since the existing RISC-V ISA cannot solve this problem, it is better
>>>> to provide some configuration for the SOC vendor to customize.
>>>
>>> We've been talking about this problem for close to five years. So no,
>>> if you don't manage to get the feature into the ISA it can't be
>>> supported.
>>
>> Isn't it a good goal for Linux to support the capabilities present in
>> the SoC that a currently being fab'd?
>>
>> I believe the CMO group only started last year [1] so the RV64GC SoCs
>> that are going into mass production this year would not have had the
>> opporuntiy of utilizing any RISC-V ISA extension for handling cache
>> management.
>
> The current Linux RISC-V policy is to only accept patches for frozen or
> ratified ISA specs.
> (Refer, Documentation/riscv/patch-acceptance.rst)
>
> This means even if emulate CMO instructions in OpenSBI, the Linux
> patches won't be taken by Palmer because CMO specification is
> still in draft stage.
>
> Also, we all know how much time it takes for RISCV international
> to freeze some spec. Judging by that we are looking at another
> 3-4 years at minimum.

Which is the root cause of most problems with riscv extension support in Linux.
All RISC-V foundation members need to apply pressure on the foundation and these
standard groups to deliver frozen specifications with an acceptable schedule.
c.f. the H extensions specs which are not yet frozen despite not having been
changed for months if not years.

--
Damien Le Moal
Western Digital Research