Re: [PATCH v7 2/4] drm/mediatek: Fix using wrong drm private data to bind mediatek-drm

[Eugen Hristev <eugen.hristev@collabora.com>]

On 7/31/23 11:21, Jason-JH Lin (林睿祥) wrote:
> Hi Eugen,
> 
> Thanks for the reviews.
> 
> On Fri, 2023-07-28 at 11:47 +0300, Eugen Hristev wrote:
>> Hi,
>>
>> On 7/27/23 19:41, Jason-JH.Lin wrote:
>>> Add checking the length of each data path before assigning drm
>>> private
>>> data into all_drm_priv array.
>>>
>>> Fixes: 1ef7ed48356c ("drm/mediatek: Modify mediatek-drm for mt8195
>>> multi mmsys support")
>>> Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
>>> ---
>>>    drivers/gpu/drm/mediatek/mtk_drm_drv.c | 16 +++++++++++++---
>>>    1 file changed, 13 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
>>> b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
>>> index 249c9fd6347e..d2fb1fb4e682 100644
>>> --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
>>> +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
>>> @@ -351,6 +351,7 @@ static bool mtk_drm_get_all_drm_priv(struct
>>> device *dev)
>>>    {
>>>    	struct mtk_drm_private *drm_priv = dev_get_drvdata(dev);
>>>    	struct mtk_drm_private *all_drm_priv[MAX_CRTC];
>>> +	struct mtk_drm_private *temp_drm_priv;
>>>    	struct device_node *phandle = dev->parent->of_node;
>>>    	const struct of_device_id *of_id;
>>>    	struct device_node *node;
>>> @@ -373,9 +374,18 @@ static bool mtk_drm_get_all_drm_priv(struct
>>> device *dev)
>>>    		if (!drm_dev || !dev_get_drvdata(drm_dev))
>>>    			continue;
>>>    
>>> -		all_drm_priv[cnt] = dev_get_drvdata(drm_dev);
>>> -		if (all_drm_priv[cnt] && all_drm_priv[cnt]-
>>>> mtk_drm_bound)
>>> -			cnt++;
>>> +		temp_drm_priv = dev_get_drvdata(drm_dev);
>>> +		if (temp_drm_priv) {
>>> +			if (temp_drm_priv->mtk_drm_bound)
>>> +				cnt++;
>>> +
>>> +			if (temp_drm_priv->data->main_len)
>>> +				all_drm_priv[0] = temp_drm_priv;
>>> +			else if (temp_drm_priv->data->ext_len)
>>> +				all_drm_priv[1] = temp_drm_priv;
>>> +			else if (temp_drm_priv->data->third_len)
>>> +				all_drm_priv[2] = temp_drm_priv;
>>> +		}
>>
>> Previously the code was assigning stuff into all_drm_priv[cnt] and
>> incrementing it.
>> With your change, it assigns to all_drm_priv[0], [1], [2]. Is this
>> what
>> you intended ?
> 
> Because dev_get_drvdata(drm_dev) will get the driver data by drm_dev.
> Each drm_dev represents a display path.
> e,g.
> drm_dev of "mediatek,mt8195-vdosys0" represents main path.
> drm_dev of "mediatek,mt8195-vdosys1" represents ext path.
> 
> So we want to make sure all_drm_priv[] store the private data in
> the order of display path, such as:
> all_drm_priv[0] = the private data of main display
> all_drm_priv[1] = the private data of ext display
> all_drm_priv[2] = the private data of third display

If you have such a hard requirement for keeping elements in an array, 
you are better having
drm_priv_main_display
drm_priv_ext_display
drm_priv_third_display

Keeping them indexed in a three elements array by having no logical 
connection between the number [0,1,2] and the actual displays that you 
want to save is a bit confusing.

One other option which I don't know if it's better or not is to have
macros to hide your indexed approach:
all_drm_priv[MAIN_DISPLAY] ...
etc.

> 
>> If this loop has second run, you will reassign to all_drm_priv again
>> ?
> 
> Because the previous code will store all_drm_priv[] in the order of
> mtk_drm_bind() was called.
> 
> If drm_dev of ext path bound earlier than drm_dev of main path,
> all_drm_priv[] in mtk_drm_get_all_drm_priv() may be re-assigned like
> this:
> all_drm_priv[0]->all_drm_priv[0] = private data of ext path
> all_drm_priv[1]->all_drm_priv[0] = private data of ext path
> all_drm_priv[0]->all_drm_priv[1] = private data of main path
> all_drm_priv[1]->all_drm_priv[1] = private data of main path
> 
> But we expect all_drm_priv[] be re-assigned like this:
> all_drm_priv[0]->all_drm_priv[0] = private data of main path
> all_drm_priv[1]->all_drm_priv[0] = private data of main path
> all_drm_priv[0]->all_drm_priv[1] = private data of ext path
> all_drm_priv[1]->all_drm_priv[1] = private data of ext path

This expectation does not appear to be really enforced in your code.
You have a driver that keeps an array with all_drm_priv[], in which
you can have main path or ext path. Then it's natural that they might 
have whichever order in the array you are placing them into.
If you have a hard enforced order of keeping elements in your array,
then an indexed array is not the best option here.
You can either: move to a different type of array , with macros for 
indexes into the array, or, store a second array/field which keeps the 
index in which you saved each element.

This is just my opinion , by looking at your code.

> 
>> I would expect you to take `cnt` into account.
>> Also, is it expected that all_drm_priv has holes in the array ?
> 
> Each drm_dev will only called mtk_drm_bind() once, so all holes
> will be filled after all drm_dev has called mtk_drm_bind().
> 
> Do you agree with this statement? :)

At the moment I cannot agree nor disagree, I don't know the code well 
enough. But what I can say, is that you should not rely on future calls 
of the function to fill up your array correctly.

> 
> Regards,
> Jason-JH.Lin
> 
>>
>> Eugen
>>
>>
>>
>>>    	}
>>>    
>>>    	if (drm_priv->data->mmsys_dev_num == cnt) {
>>
>>