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

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Date:   Tue, 11 Jul 2023 11:03:11 +0300
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     suijingfeng <suijingfeng@...ngson.cn>,
        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

On Mon, 10 Jul 2023, suijingfeng <suijingfeng@...ngson.cn> wrote:
> 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.

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@...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);
>

-- 
Jani Nikula, Intel Open Source Graphics Center