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From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     suijingfeng <suijingfeng@loongson.cn>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Li Yi <liyi@loongson.cn>
Cc:     loongson-kernel@lists.loongnix.cn, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, kernel test robot <lkp@intel.com>
Subject: Re: [PATCH] drm/loongson: Fix two warnings because of passing wrong
 type
In-Reply-To: <afbbda82-0635-bef3-b9ff-d5c6575631b8@loongson.cn>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20230710100931.255234-1-suijingfeng@loongson.cn>
 <87h6qcjc46.fsf@intel.com>
 <afbbda82-0635-bef3-b9ff-d5c6575631b8@loongson.cn>
Date:   Tue, 11 Jul 2023 11:03:11 +0300
Message-ID: <87edlej2o0.fsf@intel.com>
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On Mon, 10 Jul 2023, suijingfeng <suijingfeng@loongson.cn> wrote:
> Hi,
>
> On 2023/7/10 18:26, Jani Nikula wrote:
>> On Mon, 10 Jul 2023, Sui Jingfeng <suijingfeng@loongson.cn> wrote:
>>> When accessing I/O memory, we should pass '__iomem *' type instead of
>>> 'void *' simply, otherwise sparse tests will complain. After applied
>>> this patch, the following two sparse warnings got fixed.
>> Usually the commit message should explain why it's okay to cast away the
>> warning.
>>
>> Because realistically this doesn't "fix" the warning, this merely hides
>> it.
>
>
> My understanding is that a point itself is just a variable where store a=
=20
> address,
>
> if this address originally point to I/O memory,
>
> then, we can other cast it to u64 type, then cast it back to '__iomem *'=
=20
> again.
>
> as long as the type's=C2=A0 bit-width is width enough to store this addre=
ss,=20
> we won't lost the information.
>
>
> 'void *' or 'u64' is just a intermediate represent of the address.
>
> we can other cast it to u64 type, then cast it back to 'void __iomem *'=20
> or 'void *' again.
>
>
> Why it's okay ? My answer is that
>
> As long as a address is really point to the I/O memory, cast it to 'void=
=20
> __iomem *' is OK.
>
> As long as a address is really point to the system memory, cast it to=20
> 'void *' is OK.

The point of __iomem is to have sparse help you in tracking that, so you
don't accidentally mix up the two.

BR,
Jani.


>
>
>> BR,
>> Jani.
>>
>>> 1) drivers/gpu/drm/loongson/lsdc_benchmark.c:27:35:
>>>     sparse:     expected void volatile [noderef] __iomem *
>>>     sparse:     got void *kptr
>>>
>>> 2) drivers/gpu/drm/loongson/lsdc_benchmark.c:42:51:
>>>     sparse:     expected void const volatile [noderef] __iomem *
>>>     sparse:     got void *kptr
>>>
>>> Reported-by: kernel test robot <lkp@intel.com>
>>> Closes: https://lore.kernel.org/oe-kbuild-all/202307100243.v3hv6aes-lkp=
@intel.com/
>>> Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
>>> ---
>>>   drivers/gpu/drm/loongson/lsdc_benchmark.c | 4 ++--
>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/loongson/lsdc_benchmark.c b/drivers/gpu/dr=
m/loongson/lsdc_benchmark.c
>>> index b088646a2ff9..36e352820bdb 100644
>>> --- a/drivers/gpu/drm/loongson/lsdc_benchmark.c
>>> +++ b/drivers/gpu/drm/loongson/lsdc_benchmark.c
>>> @@ -24,7 +24,7 @@ static void lsdc_copy_gtt_to_vram_cpu(struct lsdc_bo =
*src_bo,
>>>   	lsdc_bo_kmap(dst_bo);
>>>=20=20=20
>>>   	while (n--)
>>> -		memcpy_toio(dst_bo->kptr, src_bo->kptr, size);
>>> +		memcpy_toio((void __iomem *)dst_bo->kptr, src_bo->kptr, size);
>>>=20=20=20
>>>   	lsdc_bo_kunmap(src_bo);
>>>   	lsdc_bo_kunmap(dst_bo);
>>> @@ -39,7 +39,7 @@ static void lsdc_copy_vram_to_gtt_cpu(struct lsdc_bo =
*src_bo,
>>>   	lsdc_bo_kmap(dst_bo);
>>>=20=20=20
>>>   	while (n--)
>>> -		memcpy_fromio(dst_bo->kptr, src_bo->kptr, size);
>>> +		memcpy_fromio(dst_bo->kptr, (void __iomem *)src_bo->kptr, size);
>>>=20=20=20
>>>   	lsdc_bo_kunmap(src_bo);
>>>   	lsdc_bo_kunmap(dst_bo);
>

--=20
Jani Nikula, Intel Open Source Graphics Center