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

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Date:   Wed, 16 Jun 2021 09:59:32 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     Sanjay R Mehta <sanmehta@....com>,
        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 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 k-h