linux-kernel - Re: [PATCH v2] mm/hmm/test: simplify hmm test code: use miscdevice instead of char dev

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Date:   Fri, 18 Mar 2022 04:34:20 +0200
From:   Mika Penttilä <mpenttil@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        apopple@...dia.com, jhubbard@...dia.com, rcampbell@...dia.com,
        vbabka@...e.cz
Subject: Re: [PATCH v2] mm/hmm/test: simplify hmm test code: use miscdevice
 instead of char dev



On 17.3.2022 16.15, Jason Gunthorpe wrote:
> On Thu, Mar 17, 2022 at 08:58:52AM +0200, Mika Penttilä wrote:
>> @@ -1225,7 +1232,11 @@ static int dmirror_device_init(struct dmirror_device
>> *mdevice, int id)
>>
>>          cdev_init(&mdevice->cdevice, &dmirror_fops);
>>          mdevice->cdevice.owner = THIS_MODULE;
>> -       ret = cdev_add(&mdevice->cdevice, dev, 1);
>> +       device_initialize(&mdevice->device);
>> +       dev_set_name(&mdevice->device, "%s", dmirror_device_names[id]);
>> +       mdevice->device.devt = dev;
>> +
>> +       ret = cdev_device_add(&mdevice->cdevice, &mdevice->device);
>>          if (ret)
>>                  return ret;
> 
> Right, miscdev isn't that helpful in the end..
> 
> Jason
> 

To wrap up, I could send a v3 formal patch with either this cdev way or 
the miscdev way.. Both eliminate the user space /proc/devices parsing 
and mknod'ing. Jason brought up concerns using miscdevice like this, 
although for this case it works correctly. miscdevice also provides a 
little more cleanup and simpler code, but the difference is not huge. So 
what do people prefer?

Thanks,
Mika