Subject: Re: [PATCH 03/15] powerpc/msi: Improve IRQ bitmap allocator

From: Laurentiu Tudor <[email-masked]>
Date: Mon, 22 Sep 2014 11:25:41 +0300

On 09/19/2014 11:16 PM, Scott Wood wrote:
> On Thu, 2014-09-18 at 18:26 +1000, Michael Neuling wrote:
>> From: Ian Munsie <[email-masked]>
>>
>> Currently msi_bitmap_alloc_hwirqs() will round up any IRQ allocation requests
>> to the nearest power of 2.  eg. ask for 5 IRQs and you'll get 8.  This wastes a
>> lot of IRQs which can be a scarce resource.
>>
>> For cxl we can require multiple IRQs for every contexts that is attached to the
>> accelerator.  For AFU directed accelerators, there may be 1000s of contexts
>> attached, hence we can easily run out of IRQs, especially if we are needlessly
>> wasting them.
>>
>> This changes the msi_bitmap_alloc_hwirqs() to allocate only the required number
>> of IRQs, hence avoiding this wastage.
>>
>> Signed-off-by: Ian Munsie <[email-masked]>
>> Signed-off-by: Michael Neuling <[email-masked]>
>> ---
>>  arch/powerpc/sysdev/msi_bitmap.c | 18 +++++++++++++-----
>>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> This conflicts with (and partially duplicates)
> http://patchwork.ozlabs.org/patch/381892/
> which I have in my tree.  How should we handle it?
> 
> Laurentiu, from looking at the overlap between patches I see a problem
> with your existing patch, regarding the out-of-irqs path and
> msi_bitmap_free_hwirqs(), so one way or another that needs to get fixed
> soon.
> 

Agree. My patch lacks error checking so Michael's patch is better.

---
Best Regards, Laurentiu

> 
>> diff --git a/arch/powerpc/sysdev/msi_bitmap.c b/arch/powerpc/sysdev/msi_bitmap.c
>> index 2ff6302..e001559 100644
>> --- a/arch/powerpc/sysdev/msi_bitmap.c
>> +++ b/arch/powerpc/sysdev/msi_bitmap.c
>> @@ -24,28 +24,36 @@ int msi_bitmap_alloc_hwirqs(struct msi_bitmap *bmp, int num)
>>  	 * This is fast, but stricter than we need. We might want to add
>>  	 * a fallback routine which does a linear search with no alignment.
>>  	 */
>> -	offset = bitmap_find_free_region(bmp->bitmap, bmp->irq_count, order);
>> +	offset = bitmap_find_next_zero_area(bmp->bitmap, bmp->irq_count, 0,
>> +					    num, (1 << order) - 1);
>> +	if (offset > bmp->irq_count)
>> +		goto err;
>> +	bitmap_set(bmp->bitmap, offset, num);
>>  	spin_unlock_irqrestore(&bmp->lock, flags);
>>  
>>  	pr_debug("msi_bitmap: allocated 0x%x (2^%d) at offset 0x%x\n",
>>  		 num, order, offset);
>>  
>>  	return offset;
>> +err:
>> +	spin_unlock_irqrestore(&bmp->lock, flags);
>> +	return -ENOMEM;
>>  }
>> +EXPORT_SYMBOL(msi_bitmap_alloc_hwirqs);
>>  
>>  void msi_bitmap_free_hwirqs(struct msi_bitmap *bmp, unsigned int offset,
>>  			    unsigned int num)
>>  {
>>  	unsigned long flags;
>> -	int order = get_count_order(num);
>>  
>> -	pr_debug("msi_bitmap: freeing 0x%x (2^%d) at offset 0x%x\n",
>> -		 num, order, offset);
>> +	pr_debug("msi_bitmap: freeing 0x%x at offset 0x%x\n",
>> +		 num, offset);
>>  
>>  	spin_lock_irqsave(&bmp->lock, flags);
>> -	bitmap_release_region(bmp->bitmap, offset, order);
>> +	bitmap_clear(bmp->bitmap, offset, num);
>>  	spin_unlock_irqrestore(&bmp->lock, flags);
>>  }
>> +EXPORT_SYMBOL(msi_bitmap_free_hwirqs);
>>  
>>  void msi_bitmap_reserve_hwirq(struct msi_bitmap *bmp, unsigned int hwirq)
>>  {
> 
>