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

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Date:   Wed, 16 Jun 2021 15:26:14 +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, 15:16, Sanjay R Mehta wrote:
> 
> 
> 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.

Great, but there are few more questions I had, like who creates the
device etc, can you please respond to those questions as well, so that
we understand properly how this device works

Thanks
-- 
~Vinod