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

Asutosh Das (asd)
2022-07-28T21:07:28Z

On 7/28/2022 1:29 PM, Bart Van Assche wrote:
> 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.

Thanks Bart, I wasn't aware of the background of John's Q. I will
consider your proposal and get back.

-asd