linux-kernel - Re: [PATCH 2/3] hwrng: st: Report correct FIFO size

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Message-ID: <20151006152326.GE17172@x1>
Date:	Tue, 6 Oct 2015 16:23:26 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kernel@...inux.com, herbert@...dor.apana.org.au,
	peter@...sgaard.com, festevam@...il.com, pankaj.dev@...com
Subject: Re: [PATCH 2/3] hwrng: st: Report correct FIFO size

On Tue, 06 Oct 2015, Daniel Thompson wrote:

> On 06/10/15 15:43, Lee Jones wrote:
> >The values supplied to the 'read random data from FIFO' arithmetic
> >are not correct.  The value fed in to initialise the iterator
> >describes the FIFO depth, but then the iterator is treated in
> >Bytes and subsequently increased by 2 in value for every read
> >word.  This means only 4 of the 8 available values are being read
> >during each invocation of .read().
> >
> >This change increased the device bandwidth by a factor of 2.
> >
> >Reported-by: Daniel Thompson <daniel.thompson@...aro.org>
> >Signed-off-by: Lee Jones <lee.jones@...aro.org>
> >---
> >  drivers/char/hw_random/st-rng.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/char/hw_random/st-rng.c b/drivers/char/hw_random/st-rng.c
> >index 8c8a435..44480fe 100644
> >--- a/drivers/char/hw_random/st-rng.c
> >+++ b/drivers/char/hw_random/st-rng.c
> >@@ -29,8 +29,9 @@
> >  #define ST_RNG_STATUS_BAD_ALTERNANCE	BIT(1)
> >  #define ST_RNG_STATUS_FIFO_FULL		BIT(5)
> >
> >-#define ST_RNG_FIFO_SIZE		8
> >  #define ST_RNG_SAMPLE_SIZE		2 /* 2 Byte (16bit) samples */
> >+#define ST_RNG_FIFO_DEPTH		8
> >+#define ST_RNG_FIFO_SIZE		(ST_RNG_FIFO_DEPTH * ST_RNG_SAMPLE_SIZE)
> >
> >  /* Samples are available every 0.667us, which we round to 1us */
> >  #define ST_RNG_FILL_FIFO_TIMEOUT   (1 * (ST_RNG_FIFO_SIZE / ST_RNG_SAMPLE_SIZE))
> 
> This change doubles the timeout and doesn't mention it in the
> changelog.

The next patch renders this point moot.

> Changing the order of 2/3 and 3/3 would avoid this.

Right, if this is a real concern the patches can be applied in a
different order.

> Other than that:
> 
> Reviewed-by: Daniel Thompson <daniel.thompson@...aro.org>

Thanking you.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog
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