linux-kernel - Re: [PATCH 4/4] perf: arm_spe: Enable ACPI/Platform automatic module loading

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Message-ID: <20190426080445.GA28144@fuggles.cambridge.arm.com>
Date:   Fri, 26 Apr 2019 09:04:45 +0100
From:   Will Deacon <will.deacon@....com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, catalin.marinas@....com,
        rjw@...ysocki.net, lenb@...nel.org, mark.rutland@....com,
        sudeep.holla@....com, james.morse@....com,
        lorenzo.pieralisi@....com, linuxarm@...wei.com
Subject: Re: [PATCH 4/4] perf: arm_spe: Enable ACPI/Platform automatic module
 loading

On Thu, Apr 25, 2019 at 07:58:28PM -0500, Jeremy Linton wrote:
> On 4/16/19 8:50 AM, Will Deacon wrote:
> > On Thu, Apr 04, 2019 at 12:24:38PM -0500, Jeremy Linton wrote:
> > > On 4/4/19 12:04 PM, Will Deacon wrote:
> > > > On Tue, Mar 26, 2019 at 05:39:38PM -0500, Jeremy Linton wrote:
> > > > > Lets add the MODULE_TABLE and platform id_table entries so that
> > > > > the SPE driver can attach to the ACPI platform device created by
> > > > > the core pmu code.
> > > > > 
> > > > > Signed-off-by: Jeremy Linton <jeremy.linton@....com>
> > > > > Reviewed-by: Sudeep Holla <sudeep.holla@....com>
> > > > > ---
> > > > >    drivers/perf/arm_spe_pmu.c | 11 +++++++++--
> > > > >    1 file changed, 9 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> > > > > index 7cb766dafe85..ffa2c76c08bb 100644
> > > > > --- a/drivers/perf/arm_spe_pmu.c
> > > > > +++ b/drivers/perf/arm_spe_pmu.c
> > > > > @@ -1176,7 +1176,13 @@ static const struct of_device_id arm_spe_pmu_of_match[] = {
> > > > >    };
> > > > >    MODULE_DEVICE_TABLE(of, arm_spe_pmu_of_match);
> > > > > -static int arm_spe_pmu_device_dt_probe(struct platform_device *pdev)
> > > > > +static const struct platform_device_id arm_spe_match[] = {
> > > > > +	{ "arm,spe-v1", 0},
> > > > 
> > > > It would be nice if we could avoid duplicating this string from the ACPI
> > > > parsing code.
> > > 
> > > Ok sure, I just need to find a good common place for it.
> 
> There doesn't appear to be a good common place for this, so maybe arm_pmu.h,
> which can then be included in the spe driver is the right thing.

I'm fine with that.

> > > > > +	{ }
> > > > > +};
> > > > > +MODULE_DEVICE_TABLE(platform, arm_spe_match);
> > > > > +
> > > > > +static int arm_spe_pmu_device_probe(struct platform_device *pdev)
> > > > >    {
> > > > >    	int ret;
> > > > >    	struct arm_spe_pmu *spe_pmu;
> > > > > @@ -1236,11 +1242,12 @@ static int arm_spe_pmu_device_remove(struct platform_device *pdev)
> > > > >    }
> > > > >    static struct platform_driver arm_spe_pmu_driver = {
> > > > > +	.id_table = arm_spe_match,
> > > > >    	.driver	= {
> > > > >    		.name		= DRVNAME,
> > > > >    		.of_match_table	= of_match_ptr(arm_spe_pmu_of_match),
> > > > 
> > > > Hmm, so some other drivers don't hook .id_table like you do, but instead
> > > > hook .acpi_match_table in the driver structure. Is that not better?
> > > 
> > > This isn't actually an ACPI device, (aka not defined in the namespace), so
> > > its missing much of the ACPI functionality. I think that also means its
> > > needs to be declared this way.
> > 
> > Looking at platform_match(), I'd really like to avoid having both an
> > .id_table and an .of_match_table field.
> 
> 
> > 
> > acpi_of_match_device() will actually use the .of_match_table, but it relies
> > on ACPI_COMPANION returning a valid acpi_device. If we don't have one of
> 
> Right, via the fwnode it can cause an acpi DSDT defined device with a _DSD
> "compatible" property to match an entry in the of_match_table compatible
> string. I don't think this is us...
> 
> > those, perhaps we can use the .id_table exclusively and drop the
> > .of_match_table instead?
> 
> This definitely made me do my homework, the following is AFAIK:

FWIW: I'm also feeling my way here!

> Its possible to match on just a .id_table, but this requires matching the OF
> device name against the id_table name rather than against the OF compatible
> string (*). This doesn't seem like a good idea, despite platform_device_id
> entries being significantly smaller than the of_device_id ones. Plus, I
> think we end up with two duplicate tables because we still need the
> MODULE_TABLE(of,xxx) to assure that userspace can associate the modalias
> with the module.

Well spotted, I didn't notice that the compatible string isn't used for
matching in that case.

> OTOH, it seems possible to match on module name directly ('arm_spe_pmu'),
> but this limits us to only a single device type for all ACPI device
> variations unless we put platform checks in the module itself (ick!). I
> suspect in the future if a spe.v2 were to come out this would be a problem
> unless a separate module were created. Then there is the fact this still
> needs a platform_device_id table, as the modalias will read
> "platform:arm_spe_pmu". Which will cause people to question why its not just
> assigned and matched against the .id_table.

Ok, fair enough and sorry for the wild goose chase. Looks like we'll stick
with what you had, as the alternatives all seen considerably worse.

Will