[PATCH 1/4] nvme: fix setting the queue depth in nvme_alloc_io_tag_set

[Sagi Grimberg]

>> While the CAP.MQES field in NVMe is a 0s based filed with a natural one
>> off, we also need to account for the queue wrap condition and fix undo
>> the one off again in nvme_alloc_io_tag_set.  This was never properly
>> done by the fabrics drivers, but they don't seem to care because there
>> is no actual physical queue that can wrap around, but it became a
>> problem when converting over the PCIe driver.  Also add back the
>> BLK_MQ_MAX_DEPTH check that was lost in the same commit.
>>
>> Fixes: 0da7feaa5913 ("nvme-pci: use the tagset alloc/free helpers")
> 
> This looks fine, but does this actually fix anything?
> How does nvme-pci ever set sqsize anywhere close to BLK_MQ_MAX_DEPTH ?
> NVME_PCI_MAX_QUEUE_SIZE is 4095...

Disregard this comment.

But this means that fabrics drivers automatically get 1 less queue 
entry? with this patch alone?

> 
>> Reported-by: Hugh Dickins <hughd at google.com>
>> Signed-off-by: Christoph Hellwig <hch at lst.de>
>> ---
>>   drivers/nvme/host/core.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>> index e26b085a007aea..cda1361e6d4fbb 100644
>> --- a/drivers/nvme/host/core.c
>> +++ b/drivers/nvme/host/core.c
>> @@ -4897,7 +4897,7 @@ int nvme_alloc_io_tag_set(struct nvme_ctrl 
>> *ctrl, struct blk_mq_tag_set *set,
>>       memset(set, 0, sizeof(*set));
>>       set->ops = ops;
>> -    set->queue_depth = ctrl->sqsize + 1;
>> +    set->queue_depth = min_t(unsigned, ctrl->sqsize, BLK_MQ_MAX_DEPTH 
>> - 1);
>>       /*
>>        * Some Apple controllers requires tags to be unique across 
>> admin and
>>        * the (only) I/O queue, so reserve the first 32 tags of the I/O 
>> queue.