Re: [PATCH v3 00/27] memory: omap-gpmc: mtd: nand: Support GPMC NAND on non-OMAP platforms

[Roger Quadros]

Hi Brian,

On 30/11/15 21:54, Brian Norris wrote:
> Hi Roger,
> 
> On Tue, Oct 27, 2015 at 11:37:03AM +0200, Roger Quadros wrote:
>> On 26/10/15 23:23, Brian Norris wrote:
>>> I'm not too familiar with OMAP platforms, and I might have missed out on
>>> prior discussions/context, so please forgive if I'm asking silly or old
>>> questions here.
>>
>> No worries at all.
>>
>>>
>>> On Fri, Sep 18, 2015 at 05:53:22PM +0300, Roger Quadros wrote:
>>>> - Remove NAND IRQ handling from omap-gpmc driver, share the GPMC IRQ
>>>> with the omap2-nand driver and handle NAND IRQ events in the NAND driver.
>>>> This causes performance increase when using prefetch-irq mode.
>>>> 30% increase in read, 17% increase in write in prefetch-irq mode.
>>>
>>> Have you pinpointed the exact causes for the performance increase, or
>>> can you give an educated guess? AIUI, you're reducing the number of
>>> interrupts needed for NAND prefetch mode, but you're also removing a bit
>>> of abstraction and implementing hooks that look awfully like the
>>> existing abstractions:
>>>
>>> +       int (*nand_irq_enable)(enum gpmc_nand_irq irq);
>>> +       int (*nand_irq_disable)(enum gpmc_nand_irq irq);
>>> +       void (*nand_irq_clear)(enum gpmc_nand_irq irq);
>>> +       u32 (*nand_irq_status)(void);
>>>
>>> That's not really a problem if there's a good reason for them (brcmnand
>>> implements similar hooks because of quirks in the implementation of
>>> interrupts across various BRCM SoCs, and it's not worth writing irqchip
>>> drivers for those cases). I'm mainly curious for an explanation.
>>
>> I have both implementations with me. My guess is that the 20% performance
>> gain is due to absence of irqchip/irqdomain translation code.
>> I haven't investigated further though.
> 
> I don't have much context for whether this makes sense or not. According
> to your tests, you're getting ~800K interrupts over ~15 seconds. So
> should you start noticing performance hits due to abstraction at 53K
> interrupts per second?

Yes, this was my understanding.

> 
> But anyway, I'm not sure that completely answered my question. My
> question was whether you were removing the irqchip code solely for
> performance reasons, or are there others?

Yes. Only for performance reasons.

> 
>> Another concern I have is that I'm not using any locking around
>> gpmc_nand_irq_enable/disable(). Could this pose problems in multiple NAND
>> use cases? My understanding is that it should not as the controller access
>> is serialized between multiple NAND chips.
> 
> Right, if you're touching just a NAND interrupt, and it's only used by a
> single instance of this NAND controller, then the NAND controller
> serialization code will handle this for you.

OK.

cheers,
-roger
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