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

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Message-ID: <6da1447d-01d6-222e-fc98-7e96b154d2d0@quicinc.com>
Date:   Thu, 28 Jul 2022 14:07:19 -0700
From:   "Asutosh Das (asd)" <quic_asutoshd@...cinc.com>
To:     Bart Van Assche <bvanassche@....org>,
        John Garry <john.garry@...wei.com>,
        Can Guo <quic_cang@...cinc.com>, <stanley.chu@...iatek.com>,
        <adrian.hunter@...el.com>, <alim.akhtar@...sung.com>,
        <avri.altman@....com>, <beanhuo@...ron.com>,
        <quic_nguyenb@...cinc.com>, <quic_ziqichen@...cinc.com>,
        <linux-scsi@...r.kernel.org>, <kernel-team@...roid.com>
CC:     "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Daejun Park <daejun7.park@...sung.com>,
        Jinyoung Choi <j-young.choi@...sung.com>,
        Kiwoong Kim <kwmad.kim@...sung.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] scsi: ufs: Add Multi-Circular Queue support

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