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

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Message-ID: <8e06c718-8b8a-f09a-4685-11c0c1581a0c@codeaurora.org>
Date:   Sat, 16 May 2020 15:29:19 -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 5/16/2020 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?

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