[PATCH 1/3] crypto: hw_random - Add new Exynos RNG driver
b.zolnierkie at samsung.com
Fri Mar 24 08:26:47 PDT 2017
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.
Samsung R&D Institute Poland
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