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

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Date:   Sun, 17 May 2020 13:37:20 -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 5/17/2020 1:14 AM, Greg KH wrote:
> On Sat, May 16, 2020 at 03:29:19PM -0600, Jeffrey Hugo wrote:
>> On 5/16/2020 1:01 AM, Greg KH wrote:
>>> On Fri, May 15, 2020 at 03:08:59PM -0600, Jeffrey Hugo wrote:
>>>> 2. There are a limited number of dynamic minor numbers for misc devs (64),
>>>> so if you are expecting more devices than that, a misc dev is not
>>>> appropiate.  Also, these minors are shared with other misc dev users, so
>>>> depending on the system configuration, you might have significantly less
>>>> than 64 minors available for use.
>>>
>>> I'm pretty sure we can have more than 64 misc devices, that limitation
>>> should have been removed a while ago.  Try it and see :)
>>
>> In total, there can be more tha 64 misc devices.  However my previous
>> comment was specific to dynamic minors (ie devices which do not have an
>> assigned minor).  The limit on dynamic minors still apears to be 64. Looking
>> at the code -
>>
>> DYNAMIC_MINORS is still 64
>> https://elixir.bootlin.com/linux/v5.7-rc5/source/drivers/char/misc.c#L63
>>
>> I see the same in -next
>>
>> DYNAMIC_MINORS is used to size a bitmap - one bit for each dynamic minor
>> misc device that exists at one particular point in time.  After all 64 bits
>> are consumed by misc_register() by clients requesting a dynamic minor, no
>> more dynamic minor misc devices can be registered until some are
>> unregistered.
>>
>> What am I missing?
> 
> Oops, nothing, my fault.  We fixed up the allocation of more dynamic
> majors for chardev in 2017 and for some reason I thought we also
> increased the number of misc dynamic minors at the same time, but that
> was incorrect.

No problem.

> I'll gladly take patches that bump up the number of misc minors if
> needed.

I don't think its needed at this time, but I will keep that in mind.

> But to get back to the main issue here, you are only going to have 1 or
> maybe 2 of these devices in a system at a time, right?  So "burning" a
> whole major number for that feels like a waste.

Depends on what the customer wants to do.  We support a number of 
systems, but one in particular has the capability of 6-12 devices.

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