linux-kernel - Re: [PATCH] RISC-V: Allow drivers to provide custom read

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Date:   Fri, 4 Sep 2020 22:13:18 +0530
From:   Anup Patel <anup@...infault.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Anup Patel <anup.patel@....com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <atish.patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] RISC-V: Allow drivers to provide custom read_cycles64 for
 M-mode kernel

On Fri, Sep 4, 2020 at 9:55 PM Christoph Hellwig <hch@....de> wrote:
>
> On Fri, Sep 04, 2020 at 09:51:21PM +0530, Anup Patel wrote:
> > The TIME CSR is usually not present on most RISC-V systems so the
> > M-mode firmware will emulate the TIME CSR for the S-mode (MMU) kernel
> > whereas the M-mode (NoMMU) kernel will have to use MMIO clocksource.
> >
> > Currently, the get_cycles() implementation in asm/timex.h does not
> > consider the above fact so we provide alternate implementation of
> > the get_cycles() for the M-mode (NoMMU) kernel which expects drivers
> > to provide custom MMIO based read_cycles64() method.
>
> Please just go back to the previous working version without all the
> crazy indirections.
>
> The whole timer and irq code has been turned into a giant maze of
> indirections lately.

I respectfully disagree. IMHO, the previous code made the RISC-V
timer driver convoluted (both SBI call and CLINT in one place) and
mandated CLINT for NoMMU kernel. In fact, RISC-V spec does not
mandate CLINT or PLIC. The RISC-V SOC vendors are free to
implement their own timer device, IPI device and interrupt controller.

We already have RISC-V systems (e.g. Andes AE350) where we
have a different timer and IPI device instead of CLINT.

The current code is more flexible and allows SOC vendors to write
their own timer driver under drivers/clocksource.

Regards,
Anup