Re: [PATCH v2] RISC-V: Check clint_time_val before use

[Palmer Dabbelt]

On Sat, 26 Sep 2020 03:31:29 PDT (-0700), Damien Le Moal wrote:
> On Sat, 2020-09-26 at 15:51 +0530, Anup Patel wrote:
> > The NoMMU kernel is broken for QEMU virt machine from Linux-5.9-rc6
> > because the get_cycles() and friends are called very early from
> > rand_initialize() before CLINT driver is probed. To fix this, we
> > should check clint_time_val before use in get_cycles() and friends.

I don't think this is the right way to solve that problem, as we're essentially
just lying about the timer rather than informing the system we can't get
timer-based entropy right now.  MIPS is explicit about this, I don't see any
reason why we shouldn't be as well.

Does this fix the boot issue (see below for the NULL)?  There's some other
random-related arch functions so this might not be quite the right way to do
it.

diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
index 7f659dda0032..7e39b0068932 100644
--- a/arch/riscv/include/asm/timex.h
+++ b/arch/riscv/include/asm/timex.h
@@ -33,6 +33,18 @@ static inline u32 get_cycles_hi(void)
#define get_cycles_hi get_cycles_hi
#endif /* CONFIG_64BIT */

+/*
+ * Much like MIPS, we may not have a viable counter to use at an early point in
+ * the boot process.  Unfortunately we don't have a fallback, so instead we
+ * just return 0.
+ */
+static inline unsigned long random_get_entropy(void)
+{
+	if (unlikely(clint_time_val == NULL))
+		return 0;
+	return get_cycles();
+}
+
#else /* CONFIG_RISCV_M_MODE */

static inline cycles_t get_cycles(void)

> > Fixes: d5be89a8d118 ("RISC-V: Resurrect the MMIO timer implementation
> > for M-mode systems")
> > Signed-off-by: Anup Patel <anup.patel@xxxxxxx>
> > ---
> > Changes since v1:
> >  - Explicitly initialize clint_time_val to NULL in CLINT driver to
> >    avoid hang on Kendryte K210
> > ---
> >  arch/riscv/include/asm/timex.h    | 12 +++++++++---
> >  drivers/clocksource/timer-clint.c |  2 +-
> >  2 files changed, 10 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
> > index 7f659dda0032..6e7b04874755 100644
> > --- a/arch/riscv/include/asm/timex.h
> > +++ b/arch/riscv/include/asm/timex.h
> > @@ -17,18 +17,24 @@ typedef unsigned long cycles_t;
> >  #ifdef CONFIG_64BIT
> >  static inline cycles_t get_cycles(void)
> >  {
> > -	return readq_relaxed(clint_time_val);
> > +	if (clint_time_val)
> > +		return readq_relaxed(clint_time_val);
> > +	return 0;
> >  }
> >  #else /* !CONFIG_64BIT */
> >  static inline u32 get_cycles(void)
> >  {
> > -	return readl_relaxed(((u32 *)clint_time_val));
> > +	if (clint_time_val)
> > +		return readl_relaxed(((u32 *)clint_time_val));
> > +	return 0;
> >  }
> >  #define get_cycles get_cycles
> >  
> >  static inline u32 get_cycles_hi(void)
> >  {
> > -	return readl_relaxed(((u32 *)clint_time_val) + 1);
> > +	if (clint_time_val)
> > +		return readl_relaxed(((u32 *)clint_time_val) + 1);
> > +	return 0;
> >  }
> >  #define get_cycles_hi get_cycles_hi
> >  #endif /* CONFIG_64BIT */
> > diff --git a/drivers/clocksource/timer-clint.c b/drivers/clocksource/timer-clint.c
> > index d17367dee02c..8dbec85979fd 100644
> > --- a/drivers/clocksource/timer-clint.c
> > +++ b/drivers/clocksource/timer-clint.c
> > @@ -37,7 +37,7 @@ static unsigned long clint_timer_freq;
> >  static unsigned int clint_timer_irq;
> >  
> >  #ifdef CONFIG_RISCV_M_MODE
> > -u64 __iomem *clint_time_val;
> > +u64 __iomem *clint_time_val = NULL;

This one I definately don't get.  According the internet, the C standard says

   If an object that has static storage duration is not initialized
   explicitly, it is initialized implicitly as if every member that has
   arithmetic type were assigned 0 and every member that has pointer type were
   assigned a null pointer constant.

so unless I'm missing something there shouldn't be any difference between these
two lines.  When I just apply this I get exactly the same "objdump -dt" before
and after.  I do see some difference in assembly, but only when I don't pass
"-fno-common" and that ends up being passed during my Linux builds.

> >  #endif
> >  
> >  static void clint_send_ipi(const struct cpumask *target)
>
> For Kendryte:
>
> Tested-by: Damien Le Moal <damien.lemoal@xxxxxxx>
>
> --
> Damien Le Moal
> Western Digital