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

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Date:   Wed, 16 Jun 2021 13:22:54 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Sanjay R Mehta <sanmehta@....com>
Cc:     Greg KH <gregkh@...uxfoundation.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 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)
> >>>>>> +{
> >>>>>> +     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 ?

Trying to unroll further, who creates pt_device? who creates the dev
object under this..?

Thanks
-- 
~Vinod