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Date:   Wed, 6 Dec 2023 15:02:04 -1000
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Subject: Re: [PATCH V2 1/3] ufs: core: Add CPU latency QoS support for ufs
 driver
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To:     Manivannan Sadhasivam <mani@kernel.org>,
        Naresh Maramaina <quic_mnaresh@quicinc.com>
Cc:     "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Peter Wang <peter.wang@mediatek.com>,
        Andy Gross <agross@kernel.org>,
        Bjorn Andersson <andersson@kernel.org>,
        Konrad Dybcio <konrad.dybcio@linaro.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>, chu.stanley@gmail.com,
        Alim Akhtar <alim.akhtar@samsung.com>,
        Avri Altman <avri.altman@wdc.com>, linux-scsi@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org,
        linux-arm-msm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, quic_cang@quicinc.com,
        quic_nguyenb@quicinc.com, Nitin Rawat <quic_nitirawa@quicinc.com>
References: <20231204143101.64163-1-quic_mnaresh@quicinc.com>
 <20231204143101.64163-2-quic_mnaresh@quicinc.com>
 <590ade27-b4da-49be-933b-e9959aa0cd4c@acm.org>
 <692cd503-5b14-4be6-831d-d8e9c282a95e@quicinc.com>
 <5e7c5c75-cb5f-4afe-9d57-b0cab01a6f26@acm.org>
 <b9373252-710c-4a54-95cc-046314796960@quicinc.com>
 <20231206153242.GI12802@thinkpad>
From:   Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <20231206153242.GI12802@thinkpad>
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On 12/6/23 05:32, Manivannan Sadhasivam wrote:
> On Wed, Dec 06, 2023 at 07:32:54PM +0530, Naresh Maramaina wrote:
>> On 12/5/2023 10:41 PM, Bart Van Assche wrote:
>>> On 12/4/23 21:58, Naresh Maramaina wrote:
>>>> On 12/5/2023 12:30 AM, Bart Van Assche wrote:
>>>>> On 12/4/23 06:30, Maramaina Naresh wrote:
>>>>>> +    /* This capability allows the host controller driver to
>>>>>> use the PM QoS
>>>>>> +     * feature.
>>>>>> +     */
>>>>>> +    UFSHCD_CAP_PM_QOS                = 1 << 13,
>>>>>>    };
>>>>>
>>>>> Why does it depend on the host driver whether or not PM QoS is
>>>>> enabled? Why isn't it enabled unconditionally?
>>>>
>>>> For some platform vendors power KPI might be more important than
>>>> random io KPI. Hence this flag is disabled by default and can be
>>>> enabled based on platform requirement.
>>>
>>> How about leaving this flag out unless if a host vendor asks explicitly
>>> for this flag?
>>
>> IMHO, instead of completely removing this flag, how about having
>> flag like "UFSHCD_CAP_DISABLE_PM_QOS" which will make PMQOS enable
>> by default and if some host vendor wants to disable it explicitly,
>> they can enable that flag.
>> Please let me know your opinion.

That would result in a flag that is tested but that is never set by
upstream code. I'm not sure that's acceptable.

> If a vendor wants to disable this feature, then the driver has to be modified.
> That won't be very convenient. So either this has to be configured through sysfs
> or Kconfig if flexibility matters.

Kconfig sounds worse to me because changing any Kconfig flag requires a
modification of the Android GKI kernel.

Thanks,

Bart.