linux-kernel - Re: [PATCH v1 1/2] riscv: Kconfig: Allow RV32 to build with no MMU

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Date:   Fri, 20 Jan 2023 20:44:10 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Jesse Taube <mr.bossman075@...il.com>
Cc:     Conor Dooley <conor.dooley@...rochip.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Yimin Gu <ustcymgu@...il.com>,
        Waldemar Brodkorb <wbx@...nadk.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>
Subject: Re: [PATCH v1 1/2] riscv: Kconfig: Allow RV32 to build with no MMU

Hey Jesse,

On Fri, Jan 20, 2023 at 12:39:06PM -0500, Jesse Taube wrote:
> On 1/20/23 02:59, Conor Dooley wrote:
> > Since you'll have to re-submit, making sure that allowing !MMU on rv32
> > doesn't break the build due to canaan k210 drivers being enabled despite
> > relying on 64-bit divisions, I've got some nits for you.
> Not sure what driver needs 64bit, but sense !MMU was only selected by 64BIT.

LKP reported a build error for it:
https://lore.kernel.org/linux-riscv/202301201538.zNlqgE4L-lkp@intel.com/

> This should work.
> diff --git a/arch/riscv/Kconfig.socs b/arch/riscv/Kconfig.socs
> index 69774bb362d6..b9835b8ede86 100644
> --- a/arch/riscv/Kconfig.socs
> +++ b/arch/riscv/Kconfig.socs
> @@ -43,7 +43,7 @@ config SOC_VIRT
> 
>  config SOC_CANAAN
>         bool "Canaan Kendryte K210 SoC"
> -       depends on !MMU
> +       depends on !MMU && 64BIT
>         select CLINT_TIMER if RISCV_M_MODE
>         select SERIAL_SIFIVE if TTY
>         select SERIAL_SIFIVE_CONSOLE if TTY

I don't think this is the correct fix for the problem - the drivers
really should not do implicit 64-bit divisions IMO.
Linux has division helpers for them in math64.h.
None of the other SoCs have a dependency on 64BIT and I'd not been keen
on adding on here.

I suspect the fix is as simple as the below, but I'd need to go test it.

Thanks,
Conor.

--- 8< ---
From ecfa79ad1b24f68cfccb77d666e443293d52d066 Mon Sep 17 00:00:00 2001
From: Conor Dooley <conor.dooley@...rochip.com>
Date: Fri, 20 Jan 2023 20:36:29 +0000
Subject: [PATCH] clk: k210: remove an implicit 64-bit division

The K210 clock driver depends on SOC_CANAAN, which is only selectable
when !MMU on RISC-V. !MMU is not possible on 32-bit yet, but patches
have been sent for its enabling. The kernel test robot reported this
implicit 64-bit division there.

Replace the implicit division with an explicit one.

Reported-by: kernel test robot <lkp@...el.com>
Link: https://lore.kernel.org/linux-riscv/202301201538.zNlqgE4L-lkp@intel.com/
Signed-off-by: Conor Dooley <conor.dooley@...rochip.com>
---
Since it was always guarded such that it only ever built for 64-bit, I
am not sure that a fixes tag is needed, but it would be:
Fixes: c6ca7616f7d5 ("clk: Add RISC-V Canaan Kendryte K210 clock driver")
---
 drivers/clk/clk-k210.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/clk/clk-k210.c b/drivers/clk/clk-k210.c
index 67a7cb3503c3..17c5bfb384ad 100644
--- a/drivers/clk/clk-k210.c
+++ b/drivers/clk/clk-k210.c
@@ -495,7 +495,7 @@ static unsigned long k210_pll_get_rate(struct clk_hw *hw,
 	f = FIELD_GET(K210_PLL_CLKF, reg) + 1;
 	od = FIELD_GET(K210_PLL_CLKOD, reg) + 1;
 
-	return (u64)parent_rate * f / (r * od);
+	return div_u64(parent_rate * f, r * od);
 }
 
 static const struct clk_ops k210_pll_ops = {
-- 
2.39.0


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