linux-kernel - Re: [PATCH] drm/loongson: Fix two warnings because of passing wrong type

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Date:   Mon, 10 Jul 2023 21:10:25 +0800
From:   suijingfeng <suijingfeng@...ngson.cn>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Li Yi <liyi@...ngson.cn>
Cc:     loongson-kernel@...ts.loongnix.cn, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] drm/loongson: Fix two warnings because of passing wrong
 type

Hi,

On 2023/7/10 18:26, Jani Nikula wrote:
> On Mon, 10 Jul 2023, Sui Jingfeng <suijingfeng@...ngson.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 
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 *' 
again.

as long as the type's  bit-width is width enough to store this address, 
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 *' 
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 
__iomem *' is OK.

As long as a address is really point to the system memory, cast it to 
'void *' is OK.


> 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@...el.com>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202307100243.v3hv6aes-lkp@intel.com/
>> Signed-off-by: Sui Jingfeng <suijingfeng@...ngson.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/drm/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);
>>   
>>   	while (n--)
>> -		memcpy_toio(dst_bo->kptr, src_bo->kptr, size);
>> +		memcpy_toio((void __iomem *)dst_bo->kptr, src_bo->kptr, size);
>>   
>>   	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);
>>   
>>   	while (n--)
>> -		memcpy_fromio(dst_bo->kptr, src_bo->kptr, size);
>> +		memcpy_fromio(dst_bo->kptr, (void __iomem *)src_bo->kptr, size);
>>   
>>   	lsdc_bo_kunmap(src_bo);
>>   	lsdc_bo_kunmap(dst_bo);