linux-kernel - Re: [PATCH] drm/amdgpu: Senseless code and unnecessary memset

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Date:   Wed, 6 Apr 2022 16:43:15 +0200
From:   Christian König <christian.koenig@....com>
To:     Jani Nikula <jani.nikula@...el.com>,
        Grigory Vasilyev <h0tc0d3@...il.com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        Melissa Wen <mwen@...lia.com>
Cc:     Alex Deucher <alexander.deucher@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Jiawei Gu <Jiawei.Gu@....com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/amdgpu: Senseless code and unnecessary memset

Am 06.04.22 um 16:26 schrieb Jani Nikula:
> On Tue, 05 Apr 2022, Christian König <christian.koenig@....com> wrote:
>> Am 05.04.22 um 19:36 schrieb Grigory Vasilyev:
>>> Using memset on local arrays before exiting the function is pointless.
>>> Compilator will remove this code. Also for local arrays is preferable to
>>> use {0} instead of memset. Mistakes are often made when working with
>>> memset.
>> Well actually memset is preferred when working with structures which are
>> given to the hardware parser because {0} won't initialize paddings.
> Not that I'd know anything that's going on here... but it sure seems
> strange to me to be passing unpacked structures where the padding might
> matter to a "hardware parser".

Well to me it is an absolute miracle why the heck compilers don't 
initialize paddings.

We had so many ups moments over the years and I really don't see why it 
should be more optimal to do this.

Anyway, the memset() is used intentionally here and I don't really want 
to change it. What we could do is removing the second superfluous memset 
and use sizeof() instead of hard coding the size.

Regards,
Christian.

>
> *shrug*
>
>
> BR,
> Jani.
>
>
>> So please don't use {0} in any of the atom bios code.
>>
>> Regards,
>> Christian.
>>
>>> Signed-off-by: Grigory Vasilyev <h0tc0d3@...il.com>
>>> ---
>>>    drivers/gpu/drm/amd/amdgpu/atom.c | 8 +-------
>>>    1 file changed, 1 insertion(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/atom.c b/drivers/gpu/drm/amd/amdgpu/atom.c
>>> index be9d61bcb8ae..537e48fbbe6b 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/atom.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/atom.c
>>> @@ -1538,11 +1538,9 @@ struct atom_context *amdgpu_atom_parse(struct card_info *card, void *bios)
>>>    int amdgpu_atom_asic_init(struct atom_context *ctx)
>>>    {
>>>    	int hwi = CU16(ctx->data_table + ATOM_DATA_FWI_PTR);
>>> -	uint32_t ps[16];
>>> +	uint32_t ps[16] = {0};
>>>    	int ret;
>>>    
>>> -	memset(ps, 0, 64);
>>> -
>>>    	ps[0] = cpu_to_le32(CU32(hwi + ATOM_FWI_DEFSCLK_PTR));
>>>    	ps[1] = cpu_to_le32(CU32(hwi + ATOM_FWI_DEFMCLK_PTR));
>>>    	if (!ps[0] || !ps[1])
>>> @@ -1551,10 +1549,6 @@ int amdgpu_atom_asic_init(struct atom_context *ctx)
>>>    	if (!CU16(ctx->cmd_table + 4 + 2 * ATOM_CMD_INIT))
>>>    		return 1;
>>>    	ret = amdgpu_atom_execute_table(ctx, ATOM_CMD_INIT, ps);
>>> -	if (ret)
>>> -		return ret;
>>> -
>>> -	memset(ps, 0, 64);
>>>    
>>>    	return ret;
>>>    }