Date:   Sat, 12 Aug 2023 13:31:59 +0800
From:   Barry Song <21cnbao@...il.com>
To:     Yicong Yang <yangyicong@...wei.com>
Cc:     Liang Li <liliang6@...il.cn>, 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

On Sat, Aug 12, 2023 at 1:24 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Fri, Aug 11, 2023 at 6:14 PM Yicong Yang <yangyicong@...wei.com> wrote:
> >
> > On 2023/8/9 20:11, Liang Li wrote:
> > > Hi Yicong,
> > >
> > > Thanks for your reply,
> > >
> > > On 2023-08-09 14:31, Yicong Yang <yangyicong@...wei.com> wrote:
> > >> 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.
> > >>
> > >
> > > Then what's the purpose of the added '.id_table = ...' line in the previous
> > > patch ?
> > > <We lost the patch context in this thread.>
> > >
> > > Based on above clarification, the updated DEVICE_TABLE would update modalias
> > > as expected, right ?
> >
> > ok, it's lack of illustration in the commit. If we're going to use MODULE_DEVICE_TABLE we need
> > a platform id table. So I add it and I found it weired if we have a id table but not use it for
> > probing, so I also initialize .id_table.
> >
> > I found there's also an another way to implement this by used MODULE_ALIAS(), and no need to add
> > an id table. Maybe this way is less controversial.
>
> right, how is your driver matching with your device?
>
> acpi_driver_match_device()  or strcmp(pdev->name, drv->name) == 0 ?

btw, those drivers supporting acpi probe usually have a acpi table

#if IS_ENABLED(CONFIG_ACPI)
static const struct acpi_device_id hidma_acpi_ids[] = {
{"QCOM8061"},
{"QCOM8062", HIDMA_MSI_CAP},
{"QCOM8063", (HIDMA_MSI_CAP | HIDMA_IDENTITY_CAP)},
{},
};
MODULE_DEVICE_TABLE(acpi, hidma_acpi_ids);
#endif

>
> static int platform_match(struct device *dev, struct device_driver *drv)
> {
>         struct platform_device *pdev = to_platform_device(dev);
>         struct platform_driver *pdrv = to_platform_driver(drv);
>
>         /* When driver_override is set, only bind to the matching driver */
>         if (pdev->driver_override)
>                 return !strcmp(pdev->driver_override, drv->name);
>
>         /* Attempt an OF style match first */
>         if (of_driver_match_device(dev, drv))
>                 return 1;
>
>         /* Then try ACPI style match */
>         if (acpi_driver_match_device(dev, drv))
>                 return 1;
>
>         /* Then try to match against the id table */
>         if (pdrv->id_table)
>                 return platform_match_id(pdrv->id_table, pdev) != NULL;
>
>         /* fall-back to driver name match */
>         return (strcmp(pdev->name, drv->name) == 0);
> }
>
> In both cases, it seems we don't need id_table. id_table can support
> two or above device names, so that one driver can support
> multiple device IDs usually with different specific device data.
>
> >
> > diff --git a/drivers/perf/arm_smmuv3_pmu.c b/drivers/perf/arm_smmuv3_pmu.c
> > index 25a269d431e4..4c32b6dbfe76 100644
> > --- a/drivers/perf/arm_smmuv3_pmu.c
> > +++ b/drivers/perf/arm_smmuv3_pmu.c
> > @@ -984,6 +984,7 @@ static void __exit arm_smmu_pmu_exit(void)
> >
> >  module_exit(arm_smmu_pmu_exit);
> >
> > +MODULE_ALIAS("platform:arm-smmu-v3-pmcg");
> >  MODULE_DESCRIPTION("PMU driver for ARM SMMUv3 Performance Monitors Extension");
> >  MODULE_AUTHOR("Neil Leeder <nleeder@...eaurora.org>");
> >  MODULE_AUTHOR("Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>");
> >
>
> Thanks
> Barry

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