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

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Date:   Wed, 6 Dec 2017 16:28:11 +0100
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Łukasz Stelmach <l.stelmach@...sung.com>
Cc:     robh+dt@...nel.org, Stephan Mueller <smueller@...onox.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Kukjin Kim <kgene@...nel.org>, linux-crypto@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Bartłomiej Żołnierkiewicz 
        <b.zolnierkie@...sung.com>
Subject: Re: [PATCH 1/3] crypto: exynos - Support Exynos5250+ SoCs

On Wed, Dec 6, 2017 at 3:53 PM, Łukasz Stelmach <l.stelmach@...sung.com> wrote:
> It was <2017-12-06 śro 15:05>, when Krzysztof Kozlowski wrote:
>> On Wed, Dec 6, 2017 at 2:42 PM, Łukasz Stelmach <l.stelmach@...sung.com> wrote:
>>> It was <2017-12-05 wto 14:34>, when Krzysztof Kozlowski wrote:
>>>> On Tue, Dec 5, 2017 at 1:35 PM, Łukasz Stelmach <l.stelmach@...sung.com> wrote:
>>>>> Add support for PRNG in Exynos5250+ SoCs.
>>>>>
>>>>> Signed-off-by: Łukasz Stelmach <l.stelmach@...sung.com>
>>>>> ---
>>>>>  .../bindings/crypto/samsung,exynos-rng4.txt        |  4 ++-
>>>>>  drivers/crypto/exynos-rng.c                        | 36 ++++++++++++++++++++--
>>>>>  2 files changed, 36 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git
>>>>> a/Documentation/devicetree/bindings/crypto/samsung,exynos-rng4.txt
>>>>> b/Documentation/devicetree/bindings/crypto/samsung,exynos-rng4.txt
>>>>> index 4ca8dd4d7e66..a13fbdb4bd88 100644
>>>>> --- a/Documentation/devicetree/bindings/crypto/samsung,exynos-rng4.txt
>>>>> +++ b/Documentation/devicetree/bindings/crypto/samsung,exynos-rng4.txt
>>>>> @@ -2,7 +2,9 @@ Exynos Pseudo Random Number Generator
>>>>>
>>>>>  Required properties:
>>>>>
>>>>> -- compatible  : Should be "samsung,exynos4-rng".
>>>>> +- compatible  : One of:
>>>>> +                - "samsung,exynos4-rng" for Exynos4210 and Exynos4412
>>>>> +                - "samsung,exynos5250-prng" for Exynos5250+
>>>>>  - reg         : Specifies base physical address and size of the registers map.
>>>>>  - clocks      : Phandle to clock-controller plus clock-specifier pair.
>>>>>  - clock-names : "secss" as a clock name.
>>>>> diff --git a/drivers/crypto/exynos-rng.c b/drivers/crypto/exynos-rng.c
>>>>> index 451620b475a0..894ef93ef5ec 100644
>>>>> --- a/drivers/crypto/exynos-rng.c
>>>>> +++ b/drivers/crypto/exynos-rng.c
>>>>> @@ -22,12 +22,17 @@
>>>>>  #include <linux/err.h>
>>>>>  #include <linux/io.h>
>>>>>  #include <linux/module.h>
>>>>> +#include <linux/of_device.h>
>>>>>  #include <linux/platform_device.h>
>>>>>
>>>>>  #include <crypto/internal/rng.h>
>>>>>
>>>>>  #define EXYNOS_RNG_CONTROL             0x0
>>>>>  #define EXYNOS_RNG_STATUS              0x10
>>>>> +
>>>>> +#define EXYNOS_RNG_SEED_CONF           0x14
>>>>> +#define EXYNOS_RNG_GEN_PRNG            0x02
>>>>
>>>> Use BIT(1) instead.
>
> Done.
>
>>>>> +
>>>>>  #define EXYNOS_RNG_SEED_BASE           0x140
>>>>>  #define EXYNOS_RNG_SEED(n)             (EXYNOS_RNG_SEED_BASE + (n * 0x4))
>>>>>  #define EXYNOS_RNG_OUT_BASE            0x160
>>>>> @@ -43,6 +48,11 @@
>>>>>  #define EXYNOS_RNG_SEED_REGS           5
>>>>>  #define EXYNOS_RNG_SEED_SIZE           (EXYNOS_RNG_SEED_REGS * 4)
>>>>>
>>>>> +enum exynos_prng_type {
>>>>> +       EXYNOS_PRNG_TYPE4 = 4,
>>>>> +       EXYNOS_PRNG_TYPE5 = 5,
>>>>
>>>> That's unusual numbering and naming, so just:
>>>> enum exynos_prng_type {
>>>>       EXYNOS_PRNG_EXYNOS4,
>>>>       EXYNOS_PRNG_EXYNOS5,
>>>> };
>>>>
>>>> Especially that TYPE4 and TYPE5 suggest so kind of sub-type (like
>>>> versions of some IP blocks, e.g. MFC) but it is just the family of
>>>> Exynos.
>>>
>>> Half done. I've changed TYPE to EXYNOS.
>>>
>>> I used explicit numbering in the enum because I want both values to act
>>> same true-false-wise. If one is 0 this condition is not met.
>>
>> First of all - that condition cannot happen. It is not possible from
>> the device-matching code.
>
> Let me explain what I didn't want. With the enum:
>
>     enum exynos_prng_type {
>             EXYNOS_PRNG_EXYNOS4,
>             EXYNOS_PRNG_EXYNOS5,
>     };
>
> and a code like this
>
>     if (rng->type) {
>
>     }
>
> EXYNOS_PRNG_EXYNOS4 is identical to false while EXYNOS_PRNG_EXYNPOS5
> evaluates as true. I think this is a bad idea. I don't want it ever to
> happen.
> Because chips have their own numbers I thought using those
> numbers would be OK.

It does not scale because we do not know what chips could be added...
and we might for example discover minor differences between 4210 and
4412 so you would have:
EXYNOS4 = 4
EXYNOS4412 = 5
EXYNOS5 = 6
or some other combinations confusing the reader in the future. Instead
just do not open-code the numbers because you do not need them (I mean
you do not need the exact values).

>
>> But if you want to indicate it explicitly
>> (for code reviewing?) then how about:
>> enum exynos_prng_type {
>>        EXYNOS_PRNG_UNKNOWN = 0,
>>        EXYNOS_PRNG_EXYNOS4,
>>        EXYNOS_PRNG_EXYNOS5,
>> };
>>
>> In such case you have the same effect but your intentions are clear
>> (you expect possibility of =0... which is not possible :) ).
>
> Fair enough.

Great!

>
>>>>> +               dev_info(&pdev->dev,
>>>>> +                        "Exynos Pseudo Random Number Generator (type:%d)\n",
>>>>
>>>> dev_dbg, this is not that important information to affect the boot time.
>>>
>>> Quite many devices report their presence during boot with such
>>> messages. For example:
>>>
>>> [    3.390247] exynos-ehci 12110000.usb: EHCI Host Controller
>>> [    3.395493] exynos-ehci 12110000.usb: new USB bus registered, assigned bus number 1
>>> [    3.403702] exynos-ehci 12110000.usb: irq 80, io mem 0x12110000
>>> [    3.431793] exynos-ehci 12110000.usb: USB 2.0 started, EHCI 1.00
>>>
>>> From my experience it isn't printk() itself that slows down boot but the
>>> serial console.
>>
>> True, the console is bottleneck (not necessarily serial) [1] but that
>> does not change the fact there is no need to print the type of RNG.
>
> With values of the enum not being meaningful themselves printing the
> type does not make much sense for me too. Is it ok just to print report
> the device presence?

This does not change mine arguments - this printk does not bring any
information except that there is such device. Later you might want to
print this for Exynos TRNG. Later for other module, and other. Thus I
prefer not to add it. On the other hand that is not a reason to stop
this patch so for example you could split it into separate commit. It
is kind of unrelated change and keeping it separate will not block
everything if maintainer will not want it.

Besr regards,
Krzysztof