linux-kernel - Re: [RFC PATCH v2 1/2] scsi: ufs: Add Multi-Circular Queue support

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Message-ID: <3326ad2e-e82f-7254-7e6b-bb0a120288f8@acm.org>
Date:   Thu, 25 Aug 2022 11:04:10 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     "Asutosh Das (asd)" <quic_asutoshd@...cinc.com>,
        Can Guo <quic_cang@...cinc.com>, quic_nguyenb@...cinc.com,
        quic_xiaosenh@...cinc.com, stanley.chu@...iatek.com,
        adrian.hunter@...el.com, beanhuo@...ron.com, avri.altman@....com,
        mani@...nel.org, linux-scsi@...r.kernel.org,
        kernel-team@...roid.com
Cc:     Alim Akhtar <alim.akhtar@...sung.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jinyoung Choi <j-young.choi@...sung.com>,
        jongmin jeong <jjmin.jeong@...sung.com>,
        Kiwoong Kim <kwmad.kim@...sung.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 1/2] scsi: ufs: Add Multi-Circular Queue support

On 8/24/22 18:42, Asutosh Das (asd) wrote:
> On 8/18/2022 7:41 PM, Bart Van Assche wrote:
>> On 8/11/22 03:33, Can Guo wrote:
>>> +static inline void ufshcd_mcq_process_event(struct ufs_hba *hba,
>>> +                        struct ufs_hw_queue *hwq)
>>> +{
>>> +    struct cq_entry *cqe = ufshcd_mcq_cur_cqe(hwq);
>>> +    int tag;
>>> +
>>> +    tag = ufshcd_mcq_get_tag(hba, hwq, cqe);
>>> +    ufshcd_compl_one_task(hba, tag, cqe);
>>> +}
>>
>> Consider changing "process_event" into "process_cqe". Consider 
>> renaming ufshcd_compl_one_task() into ufshcd_compl_one_cqe().
>>
> The preparatory patch that would precede this change would define 
> ufshcd_compl_one_task() in ufshcd.c. Since this function would be 
> invoked both from Single Doorbell mode and MCQ mode, 
> ufshcd_compl_one_task() sounds more relevant. What say?

The name "task" is confusing since in SCSI standard documents it refers 
to "task management" while ufshcd_compl_one_task() is not related to 
SCSI task management at all. So I would appreciate it if another name is 
chosen than ufshcd_compl_one_task().

>>> +static irqreturn_t ufshcd_handle_mcq_cq_events(struct ufs_hba *hba)
>>> +{
>>> +    struct ufs_hw_queue *hwq;
>>> +    unsigned long outstanding_cqs;
>>> +    unsigned int nr_queues;
>>> +    int i, ret;
>>> +    u32 events;
>>> +
>>> +    ret = ufshcd_vops_get_outstanding_cqs(hba, &outstanding_cqs);
>>> +    if (ret)
>>> +        outstanding_cqs = (1U << hba->nr_hw_queues) - 1;
>>> +
>>> +    /* Exclude the poll queues */
>>> +    nr_queues = hba->nr_hw_queues - hba->nr_queues[HCTX_TYPE_POLL];
>>> +    for_each_set_bit(i, &outstanding_cqs, nr_queues) {
>>> +        hwq = &hba->uhq[i];
>>> +
>>> +        events = ufshcd_mcq_read_cqis(hba, i);
>>> +        if (events)
>>> +            ufshcd_mcq_write_cqis(hba, events, i);
>>> +
>>> +        if (events & UFSHCD_MCQ_CQIS_TEPS)
>>> +            ufshcd_mcq_poll_cqe_nolock(hba, hwq);
>>> +    }
>>> +
>>> +    return IRQ_HANDLED;
>>> +}
>>
>> Why the loop over the completion queues? Shouldn't UFSHCI 4.0 
>> compliant controllers support one interrupt per completion queue?
>>
> MCQ specification doesn't define that UFSHCI 4.0 compliant HC should 
> support one interrupt per completion queue. I guess it would depend on 
> HC vendors. But it specifies ESI as an alternate method; which is 
> implemented in this patch.

It is unfortunate that support for the ESI mechanism is optional in the 
UFSHCI 4.0 specification since I consider this as one of the most 
important UFSHCI 4.0 features. I wouldn't mind if MCQ would only be 
supported for UFSHCI 4.0 controllers that support ESI.

>>> +    if (hba->nutrs != old_nutrs) {
>>> +        ufshcd_release_sdb_queue(hba, old_nutrs);
>>> +        ret = ufshcd_memory_alloc(hba);
>>> +        if (ret)
>>> +            return ret;
>>> +        ufshcd_host_memory_configure(hba);
>>> +    }
>>
>> Can this freeing + reallocating be avoided?
>>
> Umm, we thought about this. Only after reading the device params, the 
> ext_iid support and the device queue depth be determined. So didn't look 
> like there's any other way than this. If you have any ideas, please let 
> us know.