linux-kernel - Re: [PATCH v7 11/17] openrisc: account for 0 starting value in random_get

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Date:   Sun, 1 May 2022 00:34:02 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Stafford Horne <shorne@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH v7 11/17] openrisc: account for 0 starting value in
 random_get_entropy()

Hi Stafford,

On Sun, May 01, 2022 at 07:11:37AM +0900, Stafford Horne wrote:

> I was thinking about this, the reason the tick timer is returing 0 is because
> the timer is not started.  It's getting initialized right after the random
> number generator.
> 
> A patch like this helps to startup the timer during intial startup, but I am not
> sure its the best thing:
> 
> diff --git a/arch/openrisc/kernel/head.S b/arch/openrisc/kernel/head.S
> index 15f1b38dfe03..a9b3b5614e13 100644
> --- a/arch/openrisc/kernel/head.S
> +++ b/arch/openrisc/kernel/head.S
> @@ -521,6 +521,9 @@ _start:
>         l.ori   r3,r0,0x1
>         l.mtspr r0,r3,SPR_SR
>  
> +       l.movhi r3,hi(SPR_TTMR_CR)
> +       l.mtspr r0,r3,SPR_TTMR
> +
>         CLEAR_GPR(r1)
>         CLEAR_GPR(r2)
>         CLEAR_GPR(r3)

Yea, great, I was thinking about doing it in assembly earlier in boot
too, but didn't know how you'd feel about that. I like this better.

The reason I think this is a good approach is that it means the cycle
counter includes some information about how long startup takes from the
earliest stages -- which could involve probing various devices or
strange things. So enabling the timer in head.S seems good to me.

> But I wonder:
>  - Why don't any other architectures have similar issues.
>  - Is there any more correct place to do an early timer kick off.

I think most other archs (like, say, x86) have their cycle counter
enabled by default at boot time. I was surprised to see that the or1k
risc cycle counter comes disabled by default actually.

I'll send a v9 incorporating your suggested assembly change.

Jason