linux-kernel - Re: [PATCH] perf/smmuv3: Add platform id table for module auto loading

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Date:   Wed, 9 Aug 2023 14:31:09 +0800
From:   Yicong Yang <yangyicong@...wei.com>
To:     Barry Song <21cnbao@...il.com>, Liang Li <liliang6@...il.cn>
CC:     <yangyicong@...ilicon.com>, <will@...nel.org>,
        <mark.rutland@....com>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <jonathan.cameron@...wei.com>,
        <linuxarm@...wei.com>
Subject: Re: [PATCH] perf/smmuv3: Add platform id table for module auto
 loading

Hi Barry, Liang,

On 2023/8/9 13:47, Barry Song wrote:
> On Wed, Aug 9, 2023 at 1:01 PM Liang Li <liliang6@...il.cn> wrote:
>>
>> On 2023-08-07 20:22, Yicong Yang <yangyicong@...wei.com> wrote:
>>> From: Yicong Yang <yangyicong@...ilicon.com>
>>>
>>> On ACPI based system the device is probed by the name directly. If the
>>> driver is configured as module it can only be loaded manually. Add the
>>> platform id table as well as the module alias then the driver will be
>>> loaded automatically by the udev or others once the device added.
>>>
>>
>> Please consider revise the long log to clearly express the purpose of the
>> changes in this patch:
>>
>> - What's the exact issue the patch is addressing
>> - Why the changes in this patch can fix the issue or make something working
>> - Consider impact of the changes introduced by this patch
>>
>> These info may help reviewers and maintainers .. and yourself on code merge.
> 
> years ago, i found a good doc regarding this,
> https://wiki.archlinux.org/title/Modalias
> 
> guess it is because /lib/modules/$(uname -r)/modules.alias fails to contain smmu
> driver without the MODULE_DEVICE_TABLE, isn't it, yicong？

Yes I think it's the reason. I didn't find summary in kernel docs for the modalias
as well as the uevent mechanism. Arch wiki has a well illustration for the modalias
and suse[1] describes how this is used by the udev for module auto loading.

For my case I'm using a ACPI based arm64 server and after booting the arm_smmuv3_pmu.ko
is not auto loaded by the udevd since we aren't providing this information. In order
to support this we need to provide this MODULE_DEVICE_TABLE() when the smmu pmu added
as a platform device, then the userspace udev can know which module to load after the
device is added.

[1] https://documentation.suse.com/sles/15-SP1/html/SLES-all/cha-udev.html#sec-udev-drivers

Thanks.