linux-kernel - Re: [PATCH] hwrng: omap - Fix RNG wait loop timeout

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Message-ID: <CAFA6WYM4xeczVjq4wrpQ5aWvvOQMnutaQyyf+=LjoSBFeNnFmw@mail.gmail.com>
Date:   Mon, 14 Oct 2019 19:08:58 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, dsaxena@...xity.net,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Matt Mackall <mpm@...enic.com>,
        romain.perier@...e-electrons.com, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ralph Siemsen <ralph.siemsen@...aro.org>,
        Milan STEVANOVIC <milan.stevanovic@...com>,
        Ryan Harkin <ryan.harkin@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] hwrng: omap - Fix RNG wait loop timeout

On Mon, 14 Oct 2019 at 17:57, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Mon, Oct 14, 2019 at 05:32:45PM +0530, Sumit Garg wrote:
> > Existing RNG data read timeout is 200us but it doesn't cover EIP76 RNG
> > data rate which takes approx. 700us to produce 16 bytes of output data
> > as per testing results. So configure the timeout as 1000us to also take
> > account of lack of udelay()'s reliability.
>
> What "lack of udelay()'s reliability" are you concerned about?
>

For this I took reference from "drivers/char/hw_random/st-rng.c +33".
I think it's probably safe to take additional timeout rather than
relying on accuracy of udelay() based measurements. Specifically if
udelay() returns early than the expected delay (see:
include/linux/delay.h).

-Sumit

>
> Daniel.
>
> >
> > Fixes: 383212425c92 ("hwrng: omap - Add device variant for SafeXcel IP-76 found in Armada 8K")
> > Cc: <stable@...r.kernel.org>
> > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > ---
> >  drivers/char/hw_random/omap-rng.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/char/hw_random/omap-rng.c b/drivers/char/hw_random/omap-rng.c
> > index b27f396..e329f82 100644
> > --- a/drivers/char/hw_random/omap-rng.c
> > +++ b/drivers/char/hw_random/omap-rng.c
> > @@ -66,6 +66,13 @@
> >  #define OMAP4_RNG_OUTPUT_SIZE                        0x8
> >  #define EIP76_RNG_OUTPUT_SIZE                        0x10
> >
> > +/*
> > + * EIP76 RNG takes approx. 700us to produce 16 bytes of output data
> > + * as per testing results. And to account for the lack of udelay()'s
> > + * reliability, we keep the timeout as 1000us.
> > + */
> > +#define RNG_DATA_FILL_TIMEOUT                        100
> > +
> >  enum {
> >       RNG_OUTPUT_0_REG = 0,
> >       RNG_OUTPUT_1_REG,
> > @@ -176,7 +183,7 @@ static int omap_rng_do_read(struct hwrng *rng, void *data, size_t max,
> >       if (max < priv->pdata->data_size)
> >               return 0;
> >
> > -     for (i = 0; i < 20; i++) {
> > +     for (i = 0; i < RNG_DATA_FILL_TIMEOUT; i++) {
> >               present = priv->pdata->data_present(priv);
> >               if (present || !wait)
> >                       break;
> > --
> > 2.7.4
> >