linux-kernel - Re: [PATCH v9 1/3] dmaengine: ptdma: Initial driver for the AMD PTDMA

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Date:   Wed, 16 Jun 2021 15:16:00 +0530
From:   Sanjay R Mehta <sanmehta@....com>
To:     Greg KH <gregkh@...uxfoundation.org>, Vinod Koul <vkoul@...nel.org>
Cc:     Sanjay R Mehta <Sanju.Mehta@....com>, dan.j.williams@...el.com,
        Thomas.Lendacky@....com, Shyam-sundar.S-k@....com,
        Nehal-bakulchandra.Shah@....com, robh@...nel.org,
        mchehab+samsung@...nel.org, davem@...emloft.net,
        linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org
Subject: Re: [PATCH v9 1/3] dmaengine: ptdma: Initial driver for the AMD PTDMA



On 6/16/2021 1:29 PM, Greg KH wrote:
> [CAUTION: External Email]
> 
> On Wed, Jun 16, 2021 at 01:22:54PM +0530, Vinod Koul wrote:
>> On 16-06-21, 12:27, Sanjay R Mehta wrote:
>>>
>>>
>>> On 6/16/2021 11:46 AM, Greg KH wrote:
>>>> [CAUTION: External Email]
>>>>
>>>> On Wed, Jun 16, 2021 at 10:24:52AM +0530, Sanjay R Mehta wrote:
>>>>>
>>>>>
>>>>> On 6/16/2021 9:45 AM, Vinod Koul wrote:
>>>>>> [CAUTION: External Email]
>>>>>>
>>>>>> On 15-06-21, 16:50, Sanjay R Mehta wrote:
>>>>>>
>>>>>>>>> +static struct pt_device *pt_alloc_struct(struct device *dev)
> 
> In looking at this, why are you dealing with a "raw" struct device?
> Shouldn't this be a parent pointer?  Why not pass in the real type that
> this can be made a child of?
> 
> 
>>>>>>>>> +{
>>>>>>>>> +     struct pt_device *pt;
>>>>>>>>> +
>>>>>>>>> +     pt = devm_kzalloc(dev, sizeof(*pt), GFP_KERNEL);
>>>>>>>>> +
>>>>>>>>> +     if (!pt)
>>>>>>>>> +             return NULL;
>>>>>>>>> +     pt->dev = dev;
>>>>>>>>> +     pt->ord = atomic_inc_return(&pt_ordinal);
>>>>>>>>
>>>>>>>> What is the use of this number?
>>>>>>>>
>>>>>>>
>>>>>>> There are eight similar instances of this DMA engine on AMD SOC.
>>>>>>> It is to differentiate each of these instances.
>>>>>>
>>>>>> Are they individual device objects?
>>>>>>
>>>>>
>>>>> Yes, they are individual device objects.
>>>>
>>>> Then what is "ord" for?  Why are you using an atomic variable for this?
>>>> What does this field do?  Why doesn't the normal way of naming a device
>>>> come into play here instead?
>>>>
>>>
>>> Hi Greg,
>>>
>>> The value of "ord" is incremented for each device instance and then it
>>> is used to store different name for each device as shown in below snippet.
>>>
>>>     pt->ord = atomic_inc_return(&pt_ordinal);
>>>     snprintf(pt->name, MAX_PT_NAME_LEN, "pt-%u", pt->ord);
>>
>> Okay why not use device->name ?
> 
> Ah, I missed this.  Yes, do not have 2 names for the same structure,
> that is wasteful and confusing.
> 

Thanks, Greg & Vinod. I just verified with "dev_name(dev)" and this is
serving the purpose :).

I will send this change in the next version.

- Sanjay