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

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Date:   Thu, 14 May 2020 10:24:44 -0600
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     arnd@...db.de, manivannan.sadhasivam@...aro.org,
        bjorn.andersson@...aro.org, wufan@...eaurora.org,
        pratanan@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/8] qaic: Create char dev

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.