Re: [RFC PATCH 3/8] qaic: Create char dev

[Jeffrey Hugo]

On 5/14/2020 9:56 AM, Greg KH wrote:
> On Thu, May 14, 2020 at 09:05:30AM -0600, Jeffrey Hugo wrote:
> > Wow, thank you for the near immediate response.  I'm am quite impressed.
> >
> > On 5/14/2020 8:12 AM, Greg KH wrote:
> > > On Thu, May 14, 2020 at 08:07:41AM -0600, Jeffrey Hugo wrote:
> > > >    /* Copyright (c) 2019-2020, The Linux Foundation. All rights reserved. */
> > > > +#include <linux/cdev.h>
> > > > +#include <linux/idr.h>
> > > > +#include <linux/list.h>
> > > > +#include <linux/kref.h>
> > > > +#include <linux/mhi.h>
> > > >    #include <linux/module.h>
> > > >    #include <linux/msi.h>
> > > > +#include <linux/mutex.h>
> > > >    #include <linux/pci.h>
> > > >    #include <linux/pci_ids.h>
> > > > @@ -13,9 +19,242 @@
> > > >    #define PCI_DEV_AIC100			0xa100
> > > >    #define QAIC_NAME			"Qualcomm Cloud AI 100"
> > > > +#define QAIC_MAX_MINORS			256
> > >
> > > Why have a max?
> > >
> > > Why not just use a misc device so you make the logic a lot simple, no
> > > class or chardev logic to mess with at all.
> >
> > It was our understanding that the preference is not to add new misc devices.
>
> Huh, who said that?  Not the char/misc maintainer (i.e. me) :)
>
> > As I go and try to find a supporting reference for that, I cannot find one,
> > so I'm not entirely sure where that idea came from.
> >
> > In addition, we see that the Habana Labs driver also uses chardev, and has
> > chosen the same max.  We assumed that since their driver is already
> > accepted, using the same mechanisms where applicable would be the preferred
> > approach.
>
> They had good reasons why not to use a chardev and convinced me of it.
> If you can't come up with them, then stick with a misc for now please.

Interesting.  I didn't see any discussion on this.

> > Specific to the max, 256 was chosen as being a factor larger than the
> > largest system we have, therefore we figured it wouldn't be hit for a long
> > while even as we try to have a look at what might happen down the road.
> > Looking at the Habana code, it looks like they have the same value for much
> > of the same reasons, although their usecases may vary from ours somewhat.
>
> Max numbers for no good reason are not a good thing to have.
>
> > At this time, I don't think we have a strong requirement for a chardev, so
> > we could investigate a switch over to a misc dev if you would prefer that
> > over following the Habana Labs precedent.  All I ask is a confirmation that
> > is the approach you would like to see going forward after reviewing the
> > above.
>
> Please use misc.

Ok, will investigate.

--
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.