linux-kernel - Re: [PATCH] clocksource/arm_arch_timer: Enable and verify MMIO access

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Message-ID: <56ABC13B.9080500@arm.com>
Date:	Fri, 29 Jan 2016 19:44:59 +0000
From:	Robin Murphy <robin.murphy@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	mark.rutland@....com, tglx@...utronix.de,
	daniel.lezcano@...aro.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] clocksource/arm_arch_timer: Enable and verify MMIO access

On 29/01/16 18:30, Stephen Boyd wrote:
> On 01/29, Robin Murphy wrote:
>> So far, we have been blindly assuming that if a memory-mapped timer
>> frame exists, then we have access to it. Whilst it's the firmware's
>> job to give us non-secure access to frames in the first place, we
>> should not rely on it always being generous enough to also configure
>> CNTACR if it's not even using those frames itself.
>
> Hm, that first sentence is sort of misleading. We've been blindly
> assuming that the firmware has configured CNTACR to have the
> correct bits set for virtual/physical access. We've always relied
> on status = "disabled" to figure out if we can access an entire
> frame or not.

Yeah, now that I read it back that sentence is nonsense for anything 
other than the very specific ideas of "frame" and "exists" that were 
passing through my head at some point last week - how about this instead?

"So far, we have been blindly assuming that having access to a 
memory-mapped timer frame implies that the individual elements of that 
frame are already enabled."

>>
>> Explicitly enable feature-level access per-frame, and verify that the
>> access we want is really implemented before trying to make use of it.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@....com>
>> ---
>>   drivers/clocksource/arm_arch_timer.c | 39 ++++++++++++++++++++++++++++--------
>>   1 file changed, 31 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
>> index c64d543..c88485d 100644
>> --- a/drivers/clocksource/arm_arch_timer.c
>> +++ b/drivers/clocksource/arm_arch_timer.c
>> @@ -765,20 +772,34 @@ static void __init arch_timer_mem_init(struct device_node *np)
>>   	 */
>>   	for_each_available_child_of_node(np, frame) {
>>   		int n;
>> +		u32 cntacr;
>>
>>   		if (of_property_read_u32(frame, "frame-number", &n)) {
>>   			pr_err("arch_timer: Missing frame-number\n");
>> -			of_node_put(best_frame);
>>   			of_node_put(frame);
>> -			return;
>> +			goto out;
>>   		}
>>
>> -		if (cnttidr & CNTTIDR_VIRT(n)) {
>> +		/* Try enabling everything, and see what sticks */
>> +		cntacr = CNTACR_RFRQ | CNTACR_RWPT | CNTACR_RPCT |
>> +			 CNTACR_RWVT | CNTACR_RVOFF | CNTACR_RVCT;
>> +		writel_relaxed(cntacr, cntctlbase + CNTACR(n));
>> +		cntacr = readl_relaxed(cntctlbase + CNTACR(n));
>> +
>> +		if (~cntacr & CNTACR_RFRQ)
>> +			continue;
>
> Do we need this check? If we can't read the frequency we fall
> back to looking for the DT property, so it shouldn't matter if we
> can't read the hardware there.

I was really just playing safe to start with. If we don't have cause to 
care about the difference between not having access vs. not having a 
frequency programmed then I'd agree it can probably go.

>> +
>> +		if ((cnttidr & CNTTIDR_VIRT(n)) &&
>> +		    !(~cntacr & (CNTACR_RWVT | CNTACR_RVCT))) {
>>   			of_node_put(best_frame);
>>   			best_frame = frame;
>>   			arch_timer_mem_use_virtual = true;
>>   			break;
>>   		}
>> +
>> +		if (~cntacr & (CNTACR_RWPT | CNTACR_RPCT))
>> +			continue;
>> +
>>   		of_node_put(best_frame);
>>   		best_frame = of_node_get(frame);
>>   	}
>
> Otherwise the patch looks fine and passes some light testing on
> qcom devices.

Great, thanks. The Trusted Firmware guys' warning shot has gone upstream 
already, if it helps:

https://github.com/ARM-software/arm-trusted-firmware/commit/01fc3f7300e86b0b672977133c3028d638d0c672

> BTW, I'd like to add this patch on top so that we get some info
> in /proc/iomem about which frame region is in use.

It's about time I educated myself on what all the resource stuff really 
means, but superficially it seems reasonable, and it certainly makes the 
expected "2a830000-2a83ffff : arch_mem_timer" show up on my Juno.

Thanks,
Robin.

> ---8<---
> From: Stephen Boyd <sboyd@...eaurora.org>
> Subject: [PATCH] clocksource/arm_arch_timer: Map frame with
>   of_io_request_and_map()
>
> Let's use the of_io_request_and_map() API so that the frame
> region is protected and shows up in /proc/iomem.
>
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
>   drivers/clocksource/arm_arch_timer.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index c88485d489bf..59a08fd4f76a 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -804,7 +804,8 @@ static void __init arch_timer_mem_init(struct device_node *np)
>   		best_frame = of_node_get(frame);
>   	}
>
> -	base = arch_counter_base = of_iomap(best_frame, 0);
> +	base = arch_counter_base = of_io_request_and_map(best_frame, 0,
> +							 "arch_mem_timer");
>   	if (!base) {
>   		pr_err("arch_timer: Can't map frame's registers\n");
>   		goto out;
>