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

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Message-ID: <12ff7989-c89d-d220-da23-c13ddc53384e@amd.com>
Date:   Wed, 16 Jun 2021 12:27:32 +0530
From:   Sanjay R Mehta <sanmehta@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Vinod Koul <vkoul@...nel.org>,
        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 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)
>>>>>> +{
>>>>>> +     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);


- Sanjay