Re: [PATCH] RISC-V: Allow drivers to provide custom read_cycles64 for M-mode kernel

[Anup Patel]

On Fri, Sep 4, 2020 at 9:55 PM Christoph Hellwig <hch@xxxxxx> 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