Re: [PATCH] crypto: crc32: make crc32 available in PBL

[Johannes Zink <j.zink@pengutronix.de>]

Hi Ahmad,

On 8/29/23 13:08, Ahmad Fatoum wrote:
> On 29.08.23 12:55, Johannes Zink wrote:
>> On 8/29/23 12:45, Ahmad Fatoum wrote:
>>>>> +lwl-$(CONFIG_CRC32_EARLY)        += crc32.o
>>>>
>>>> pbl-obj- is the correct prefix. lwl- means pbl- if we have PBL
>>>> support at all and obj- otherwise (for legacy systems without PBL),
>>>> while pbl-obj- is equivalent to duplicating the line once with pbl-
>>>> and once with obj-
>>>
>>> s/pbl-obj-/obj-pbl-/ :)
> 
> Sorry, had a small brain fart here.
> 
> You didn't remove the original obj-,
> so now lwl- either expands to and extra obj- or to pbl-.
> 
> obj-pbl- makes sense when you use the same symbol for both PBL and
> barebox proper, but as you're introducing a new symbol, you can
> leave it as lwl- or make it pbl- for explicitness.
> 

ok. I think I won't glob it in obj-pbl, as one might want to explicitly add 
crc32.o only for PBL or only for barebox-proper.

>> ack, gonna fix this for v2.
>>
>>>>
>>>>>    obj-$(CONFIG_DIGEST_SHA384_GENERIC)    += sha4.o
>>>>>    obj-$(CONFIG_DIGEST_SHA512_GENERIC)    += sha4.o
>>>>>    obj-y    += memneq.o
>>>>> diff --git a/crypto/crc32.c b/crypto/crc32.c
>>>>> index 95cb2212db2b..284d39351682 100644
>>>>> --- a/crypto/crc32.c
>>>>> +++ b/crypto/crc32.c
>>>>> @@ -22,7 +22,7 @@
>>>>>    #define STATIC static inline
>>>>>    #endif
>>>>>    -#ifdef CONFIG_DYNAMIC_CRC_TABLE
>>>>> +#if defined(CONFIG_DYNAMIC_CRC_TABLE) && !defined(__PBL__)
>>>>
>>>> You could also replace the dynamic allocation with a static array initialized
>>>> to zero. That way you can have a dynamic crc table even in PBL without affecting
>>>> image size as the BSS is not part of the image.
>>
>> ack. Is this ok?
>>
>> #ifdef __PBL__
>>    static uint32_t _crc_table_memory[sizeof(uint32_t) * 256] = { 0 };
> 
> The array is 256 elements, not 1024 elements. Explicit intialization
> is unnecessary.

good catch. thanks.

> 
>> #endif
>>
>> static void *alloc_crc_table() {
>> #ifdef __PBL__
>>      return _crc_table_memory;
>> #else
>>      return xmalloc(sizeof(uint32_t) * 256);
>> #endif
>> }
>>
>> If so, I can change it for v2.
> 
> My idea was to drop the allocation altogether by using BSS.
> If you do this, you should not need any __PBL__ checking at all.

I guess that should work indeed. Do I need to explicitly mark the array with a 
prefix to force it into BSS (sorry, I am not too familiar with the barebox 
linker scripts...)?

> 
> Either you have CONFIG_DYNAMIC_CRC_TABLE and the table is dynamically
> filled in bss on first access or you have CONFIG_DYNAMIC_CRC_TABLE=n
> and the table is already there and need not be allocated.
> 
> On a second thought, I am not sure if we want a table at all in PBL.
> Do you do a lot of CRC32 computation? Maybe we should just not use
> a table at all in PBL and just calculate a single crc32?
> That's what Sascha did here:
> 
> 2d13b856604b ("crc: Add PBL variant for crc_itu_t()")
> 

For my specific usecase I need to check a single CRC32 once, so probably there 
is the same or even more overhead to dynamically filling the Table than just 
doing a single CRC32 computation. Maybe more overhead than using the 
precompiled, pre-filled table, but I fully acknowledge that size matters in PBL.

I like Sascha's approach, so let me try to do a dynamic computation for the PBL 
implementation in my v2 patch.

Best regards
Johannes

> Let me know what you think.
> 
> Cheers,
> Ahmad
> 
>>
>> Best regards
>> Johannes
>>
>>
>>>>
>>>>>      static uint32_t *crc_table;
>>>>>    @@ -145,7 +145,7 @@ STATIC uint32_t crc32(uint32_t crc, const void *_buf, unsigned int len)
>>>>>    {
>>>>>        const unsigned char *buf = _buf;
>>>>>    -#ifdef CONFIG_DYNAMIC_CRC_TABLE
>>>>> +#if defined(CONFIG_DYNAMIC_CRC_TABLE) && !defined(__PBL__)
>>>>>        if (!crc_table)
>>>>>            make_crc_table();
>>>>>    #endif
>>>>> @@ -171,7 +171,7 @@ STATIC uint32_t crc32_no_comp(uint32_t crc, const void *_buf, unsigned int len)
>>>>>    {
>>>>>       const unsigned char *buf = _buf;
>>>>>    -#ifdef CONFIG_DYNAMIC_CRC_TABLE
>>>>> +#if defined(CONFIG_DYNAMIC_CRC_TABLE) && !defined(__PBL__)
>>>>>        if (!crc_table)
>>>>>            make_crc_table();
>>>>>    #endif
>>>>>
>>>>> ---
>>>>> base-commit: bef38b18eeb5d2f1fac334fb8b831e47261e099c
>>>>> change-id: 20230829-crc32_in_pbl-4d824629d4e2
>>>>>
>>>>> Best regards,
>>>>
>>>
>>
> 

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