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Date:   Fri, 29 Jul 2022 09:43:21 -0700
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Subject: Re: [PATCH 1/2] scsi: ufs: Add Multi-Circular Queue support
From:   "Asutosh Das (asd)" <quic_asutoshd@quicinc.com>
To:     Bart Van Assche <bvanassche@acm.org>,
        John Garry <john.garry@huawei.com>,
        Can Guo <quic_cang@quicinc.com>, <stanley.chu@mediatek.com>,
        <adrian.hunter@intel.com>, <alim.akhtar@samsung.com>,
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        <quic_nguyenb@quicinc.com>, <quic_ziqichen@quicinc.com>,
        <linux-scsi@vger.kernel.org>, <kernel-team@android.com>
CC:     "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Daejun Park <daejun7.park@samsung.com>,
        Jinyoung Choi <j-young.choi@samsung.com>,
        Kiwoong Kim <kwmad.kim@samsung.com>,
        open list <linux-kernel@vger.kernel.org>
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Hi Bart,

On 7/28/2022 2:07 PM, Asutosh Das (asd) wrote:
> 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.
> 

I went through the change and fwiu of your proposal is to use 
host_tagset = 1 if HC doesn't support EXT_IID capability.
That way tags would be shared across 16 HWQs (4-bit IID encoding the 
queue-ids).
That would also mean that the hba->nutrs would have to be adjusted such 
that the tags(8-bit) don't exceed 255.

Summarily,
if EXT_IID is not supported:
host_tagset = 1, maximum configurable hba->nutrs = 16, maximum 
configurable nr_hw_queues = 16.
maximum number of outstanding commands to host = 16 x 16 = 256.

if EXT_IID is supported:
host_tagset = 0, maximum confiugrable hba-nutrs = 255, maximum 
configurable nr_hw_queues = 255.

Please let me know if I'm missing something.

Thank you,
-asd