linux-kernel - Re: [PATCH 2/2] Staging: comedi: comedi_fops: Fix "out of minor numbers for board device files"

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Message-ID: <20170308100809.GB2496@linux-Precision-WorkStation-T5500>
Date:   Wed, 8 Mar 2017 18:08:09 +0800
From:   Cheah Kok Cheong <thrust73@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     abbotti@....co.uk, hsweeten@...ionengravers.com,
        gregkh@...uxfoundation.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] Staging: comedi: comedi_fops: Fix "out of minor
 numbers for board device files"

Dear Dan,
  Thanks for reviewing this patch.

On Wed, Mar 08, 2017 at 08:54:47AM +0300, Dan Carpenter wrote:
> On Sun, Mar 05, 2017 at 03:22:33AM +0800, Cheah Kok Cheong wrote:
> > If comedi module is loaded with the following max allowed parameter
> > [comedi_num_legacy_minors=48], subsequent loading of an auto-configured
> > device will fail.
> 
> Don't set comedi_num_legacy_minors=48, then?
> 
> This doesn't seem like the right fix at all.  Why only allow one auto
> configured board?  Why not 5 or 10?
> 

Let me explain, the original intended behaviour is to allow user to
reserve up to 48 minor numbers for legacy devices.

Therefore [sudo modprobe comedi comedi_num_legacy_minors=3] 
will allocate minor number 0, 1, 2 for legacy devices.
Subsequent loading of an auto-configured device will use minor number 3.
And the next one number 4 so on and so forth.

Now for the corner case of [comedi_num_legacy_minors=48] which
is supposed to reserve minor number 0 till 47 for legacy devices,
and is supposed to allocate number 48 and so on for auto-configured
devices, does not allocate number 48 anymore after commit
38b9722a4414.

This is due to the changes in comedi_alloc_board_minor().

As to why I chose to limit [comedi_num_legacy_minors=47], is given
in the commit log.

This will allow user to allocate 0 till 46 for legacy devices and
subsequent auto-configured devices will start from 47 and so forth.

I don't think anybody will miss one less number for legacy devices
otherwise there'll be complains earlier on.

Thanks.

Brgds,
CheahKC 

> regards,
> dan carpenter
>