linux-kernel - Re: [PATCH v5 05/10] xen/blkfront: negotiate number of queues/rings to be used with backend

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Message-ID: <564A631E.7040905@oracle.com>
Date:	Tue, 17 Nov 2015 07:13:34 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
	roger.pau@...rix.com, felipe.franciosi@...rix.com, axboe@...com,
	avanzini.arianna@...il.com, rafal.mielniczuk@...rix.com,
	jonathan.davies@...rix.com, david.vrabel@...rix.com
Subject: Re: [PATCH v5 05/10] xen/blkfront: negotiate number of queues/rings
 to be used with backend


On 11/17/2015 05:27 AM, Konrad Rzeszutek Wilk wrote:
>>  /* Common code used when first setting up, and when resuming. */
>>  static int talk_to_blkback(struct xenbus_device *dev,
>> @@ -1527,10 +1582,9 @@ static int talk_to_blkback(struct xenbus_device *dev,
>>  {
>>  	const char *message = NULL;
>>  	struct xenbus_transaction xbt;
>> -	int err, i;
>> -	unsigned int max_page_order = 0;
>> +	int err;
>> +	unsigned int i, max_page_order = 0;
>>  	unsigned int ring_page_order = 0;
>> -	struct blkfront_ring_info *rinfo;
> 
> Why? You end up doing the 'struct blkfront_ring_info' decleration
> in two of the loops below?

Oh, that's because Roger mentioned we would be tempted to declare rinfo only inside the for loop, to limit
the scope.

>>  
>>  	err = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
>>  			   "max-ring-page-order", "%u", &max_page_order);
>> @@ -1542,7 +1596,8 @@ static int talk_to_blkback(struct xenbus_device *dev,
>>  	}
>>  
>>  	for (i = 0; i < info->nr_rings; i++) {
>> -		rinfo = &info->rinfo[i];
>> +		struct blkfront_ring_info *rinfo = &info->rinfo[i];
>> +
> 
> Here..
> 
>> @@ -1617,7 +1677,7 @@ again:
>>  
>>  	for (i = 0; i < info->nr_rings; i++) {
>>  		int j;
>> -		rinfo = &info->rinfo[i];
>> +		struct blkfront_ring_info *rinfo = &info->rinfo[i];
> 
> And here?
> 
> It is not a big deal but I am curious of why add this change?
> 
>> @@ -1717,7 +1789,6 @@ static int blkfront_probe(struct xenbus_device *dev,
>>  
>>  	mutex_init(&info->mutex);
>>  	spin_lock_init(&info->dev_lock);
>> -	info->xbdev = dev;
> 
> That looks like a spurious change? Ah, I see that we do the same exact
> operation earlier in the blkfront_probe.
> 

The place of this line was changed because:

1738         info->xbdev = dev;

1739         /* Check if backend supports multiple queues. */
1740         err = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
                                         	^^^^
						We need xbdev to be set in advance.
                                  
1741                            "multi-queue-max-queues", "%u", &backend_max_queues);
1742         if (err < 0)
1743                 backend_max_queues = 1;

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