linux-kernel - Re: [PATCH v2 2/2] arm: perf: Mark as non-removable

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Date:   Wed, 04 Jan 2017 10:19:46 +0100
From:   Alexander Stein <alexander.stein@...tec-electronic.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/2] arm: perf: Mark as non-removable

Hi,

On Thursday 22 December 2016 22:48:32, Mark Rutland wrote:
> On Wed, Dec 21, 2016 at 04:03:40PM +0100, Alexander Stein wrote:
> > This driver can only built into the kernel. So disallow driver bind/unbind
> > and also prevent a kernel error in case DEBUG_TEST_DRIVER_REMOVE is
> > enabled.
> > 
> > Signed-off-by: Alexander Stein <alexander.stein@...tec-electronic.com>
> > ---
> > 
> >  arch/arm/kernel/perf_event_v7.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/arm/kernel/perf_event_v7.c
> > b/arch/arm/kernel/perf_event_v7.c index b942349..795e373 100644
> > --- a/arch/arm/kernel/perf_event_v7.c
> > +++ b/arch/arm/kernel/perf_event_v7.c
> > @@ -2029,6 +2029,7 @@ static int armv7_pmu_device_probe(struct
> > platform_device *pdev)> 
> >  static struct platform_driver armv7_pmu_driver = {
> >  
> >  	.driver		= {
> >  	
> >  		.name	= "armv7-pmu",
> > 
> > +		.suppress_bind_attrs = true,
> > 
> >  		.of_match_table = armv7_pmu_of_device_ids,
> >  	
> >  	},
> 
> While this patch looks correct, the other perf_event_* drivers (e.g. those
> under arch/arm/) will need similar treatment.

Yep, but this is the only one I can actually test.

> More generally, updating each and every driver in this manner seems like a
> scattergun approach that is tiresome and error prone.
> 
> IMO, it would be vastly better for a higher layer to enforce that we don't
> attempt to unbind drivers where the driver does not have a remove callback,
> as is the case here (and I suspect most over cases where
> DEBUG_TEST_DRIVER_REMOVE is blowing up).

You mean something like this?
> diff --git a/drivers/base/driver.c b/drivers/base/driver.c
> index 4eabfe2..3b6c1a2d 100644
> --- a/drivers/base/driver.c
> +++ b/drivers/base/driver.c
> @@ -158,6 +158,9 @@ int driver_register(struct device_driver *drv)
> 
>                 printk(KERN_WARNING "Driver '%s' needs updating - please use
>                 "
>                 
>                         "bus_type methods\n", drv->name);
> 
> +       if (!drv->remove)
> +               drv->suppress_bind_attrs = true;
> +
> 
>         other = driver_find(drv->name, drv->bus);
>         if (other) {
>         
>                 printk(KERN_ERR "Error: Driver '%s' is already registered, "

> Is there any reason that can't be enforced at the bus layer, say?

I'm not sure if the change above works with remove functions set in struct 
bus_type too.
But on the other hand this would hide errors in drivers which are actually 
removable but do not cleanup properly which DEBUG_TEST_DRIVER_REMOVE tries to 
detect. By setting .suppress_bind_attrs = true explicitely you state "This 
driver cannot be removed!", so the remove callback is not missing by accident.

Best regards,
Alexander