linux-kernel - Re: [PATCH 1/3] crypto: hw_random

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Message-id: <3940941.n3qdgn1RbN@amdc3058>
Date:   Fri, 24 Mar 2017 16:26:47 +0100
From:   Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Kukjin Kim <kgene@...nel.org>,
        Javier Martinez Canillas <javier@....samsung.com>,
        Matt Mackall <mpm@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-crypto@...r.kernel.org, Olof Johansson <olof@...om.net>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 1/3] crypto: hw_random - Add new Exynos RNG driver


Hi,

Firstly, thanks for working on this.

The patch looks fine overall for me, some review comments below.

On Friday, March 24, 2017 05:24:44 PM Krzysztof Kozlowski wrote:
> Replace existing hw_ranndom/exynos-rng driver with a new, reworked one.
> This is a driver for pseudo random number generator block which on
> Exynos4 chipsets must be seeded with some value.  On newer Exynos5420
> chipsets it might seed itself from true random number generator block
> but this is not implemented yet.
> 
> New driver is a complete rework to use the crypto ALGAPI instead of
> hw_random API.  Rationale for the change:
> 1. hw_random interface is for true RNG devices.
> 2. The old driver was seeding itself with jiffies which is not a
>    reliable source for randomness.
> 3. Device generates five random numbers in each pass but old driver was
>    returning only one thus its performance was reduced.
> 
> Compatibility with DeviceTree bindings is preserved.
> 
> New driver does not use runtime power management but manually enables
> and disables the clock when needed.  This is preferred approach because
> using runtime PM just to toggle clock is huge overhead.  Another

I'm not entirely convinced that the new approach is better.

With the old approach exynos_rng_generate() can be called more
than once before PM autosuspend kicks in and thus clk_prepare_enable()/
clk_disable()_unprepare() operations will be done only once. This
would give better performance on the "burst" operations.

[ The above assumes that clock operations are more costly than
  going through PM core to check the current device state. ]

> +static int exynos_rng_get_random(struct exynos_rng_dev *rng,
> +				 u8 *dst, unsigned int dlen,
> +				 unsigned int *read)
> +{
> +	int retry = 100;

I know that this is copied verbatim from the old driver but please
use define for the maximum number of retries.

> +static int exynos_rng_probe(struct platform_device *pdev)
> +{
> +	struct exynos_rng_dev *rng;
> +	struct resource *res;
> +	int ret;
> +
> +	if (exynos_rng_dev)
> +		return -EEXIST;

How this condition could ever happen? 

The probe function will never be called twice.

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics