Re: [PATCH 1/2] scsi: ufs: Add Multi-Circular Queue support

[Bart Van Assche <bvanassche@acm.org>]

On 7/28/22 12:15, Asutosh Das (asd) wrote:
> Hello John,
> 
> On 7/28/2022 12:10 PM, John Garry wrote:
>> On 19/07/2022 08:01, Can Guo wrote:
>>> +
>>> +    hba->nr_queues[HCTX_TYPE_DEFAULT] = num_possible_cpus();
>>> +    hba->nr_queues[HCTX_TYPE_READ] = 0;
>>> +    hba->nr_queues[HCTX_TYPE_POLL] = 1;
>>> +
>>> +    for (i = 0; i < HCTX_MAX_TYPES; i++)
>>> +        host->nr_hw_queues += hba->nr_queues[i];
>>> +
>>> +    host->can_queue = hba->nutrs;
>>> +    host->cmd_per_lun = hba->nutrs;
>>> +
>>> +    /* One more reserved for dev_cmd_queue */
>>> +    hba->nr_hw_queues = host->nr_hw_queues + 1;
>>> +
>>
>> So this would mean that the host can accept .can_queue * .nr_hw_queues 
>> numbers of requests simultaneously - is that true?
>
> That would mean that .can_queue * .nr_hw_queues numbers of request may 
> be queued to the host.
> Please can you elaborate if you see an issue.

Hi Asutosh,

The `host_tagset` flag has been introduced by John and Hannes some time 
ago. See also commit bdb01301f3ea ("scsi: Add host and host template 
flag 'host_tagset'"). This flag supports sharing tags across hardware 
queues and could be used to support UFSHCI 4.0 controllers that do not 
support the EXT_IID feature.

In order not to complicate the implementation further, I propose to fall 
back to the UFSHCI 3.0 compatibility mode for UFSHCI 4.0 controllers 
that do not support the EXT_IID feature.

To answer John's question: the maximum number of outstanding commands is 
16 * hba->nutrs if EXT_IID is supported (EXT_IID is a four bits field). 
If the hardware queue index is encoded in the EXT_IID field, hba->nutrs 
is the number of commands per hardware queue.

Thanks,

Bart.