Re: [v4 07/11] soc/fsl/qbman: Rework portal mapping calls for ARM/PPC

[Roy Pledge]

On 9/15/2017 5:49 PM, Catalin Marinas wrote:
> On Thu, Sep 14, 2017 at 07:07:50PM +0000, Roy Pledge wrote:
>> On 9/14/2017 10:00 AM, Catalin Marinas wrote:
>>> On Thu, Aug 24, 2017 at 04:37:51PM -0400, Roy Pledge wrote:
>>>> @@ -123,23 +122,34 @@ static int bman_portal_probe(struct platform_device *pdev)
>>>>    	}
>>>>    	pcfg->irq = irq;
>>>>    
>>>> -	va = ioremap_prot(addr_phys[0]->start, resource_size(addr_phys[0]), 0);
>>>> -	if (!va) {
>>>> -		dev_err(dev, "ioremap::CE failed\n");
>>>> +	/*
>>>> +	 * TODO: Ultimately we would like to use a cacheable/non-shareable
>>>> +	 * (coherent) mapping for the portal on both architectures but that
>>>> +	 * isn't currently available in the kernel.  Because of HW differences
>>>> +	 * PPC needs to be mapped cacheable while ARM SoCs will work with non
>>>> +	 * cacheable mappings
>>>> +	 */
>>>
>>> This comment mentions "cacheable/non-shareable (coherent)". Was this
>>> meant for ARM platforms? Because non-shareable is not coherent, nor is
>>> this combination guaranteed to work with different CPUs and
>>> interconnects.
>>
>> My wording is poor I should have been clearer that non-shareable ==
>> non-coherent.  I will fix this.
>>
>> We do understand that cacheable/non shareable isn't supported on all
>> CPU/interconnect combinations but we have verified with ARM that for the
>> CPU/interconnects we have integrated QBMan on our use is OK. The note is
>> here to try to explain why the mapping is different right now. Once we
>> get the basic QBMan support integrated for ARM we do plan to try to have
>> patches integrated that enable the cacheable mapping as it gives a
>> significant performance boost.
> 
> I will definitely not ack those patches (at least not in the form I've
> seen, assuming certain eviction order of the bytes in a cacheline). The
> reason is that it is incredibly fragile, highly dependent on the CPU
> microarchitecture and interconnects. Assuming that you ever only have a
> single SoC with this device, you may get away with #ifdefs in the
> driver. But if you support two or more SoCs with different behaviours,
> you'd have to make run-time decisions in the driver or run-time code
> patching. We are very keen on single kernel binary image/drivers and
> architecturally compliant code (the cacheable mapping hacks are well
> outside the architecture behaviour).
> 

Let's put this particular point on hold for now, I would like to focus 
on getting the basic functions merged in ASAP. I removed the comment in 
question (it sort of happened naturally when I applied your other 
comments) in the next revision of the patchset.  I have submitted the 
patches to our automated test system for sanity checking and I will sent 
a new patchset once I get the results.

Thanks again for your comments - they have been very useful and have 
improved the quality of the code for sure.

>>>> diff --git a/drivers/soc/fsl/qbman/dpaa_sys.h b/drivers/soc/fsl/qbman/dpaa_sys.h
>>>> index 81a9a5e..0a1d573 100644
>>>> --- a/drivers/soc/fsl/qbman/dpaa_sys.h
>>>> +++ b/drivers/soc/fsl/qbman/dpaa_sys.h
>>>> @@ -51,12 +51,12 @@
>>>>    
>>>>    static inline void dpaa_flush(void *p)
>>>>    {
>>>> +	/*
>>>> +	 * Only PPC needs to flush the cache currently - on ARM the mapping
>>>> +	 * is non cacheable
>>>> +	 */
>>>>    #ifdef CONFIG_PPC
>>>>    	flush_dcache_range((unsigned long)p, (unsigned long)p+64);
>>>> -#elif defined(CONFIG_ARM)
>>>> -	__cpuc_flush_dcache_area(p, 64);
>>>> -#elif defined(CONFIG_ARM64)
>>>> -	__flush_dcache_area(p, 64);
>>>>    #endif
>>>>    }
>>>
>>> Dropping the private API cache maintenance is fine and the memory is WC
>>> now for ARM (mapping to Normal NonCacheable). However, do you require
>>> any barriers here? Normal NC doesn't guarantee any ordering.
>>
>> The barrier is done in the code where the command is formed. We follow
>> this pattern
>> a) Zero the command cache line (the device never reacts to a 0 command
>> verb so a cast out of this will have no effect)
>> b) Fill in everything in the command except the command verb (byte 0)
>> c) Execute a memory barrier
>> d) Set the command verb (byte 0)
>> e) Flush the command
>> If a castout happens between d) and e) doesn't matter since it was about
>> to be flushed anyway .  Any castout before d) will not cause HW to
>> process the command because verb is still 0. The barrier at c) prevents
>> reordering so the HW cannot see the verb set before the command is formed.
> 
> I think that's fine, the dpaa_flush() can be a no-op with non-cacheable
> memory (I had forgotten the details).
>