linux-kernel - Re: [PATCH V2 1/3] ufs: core: Add CPU latency QoS support for ufs driver

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Date:   Thu, 7 Dec 2023 15:56:43 +0530
From:   Nitin Rawat <quic_nitirawa@...cinc.com>
To:     Manivannan Sadhasivam <mani@...nel.org>,
        Bart Van Assche <bvanassche@....org>
CC:     Naresh Maramaina <quic_mnaresh@...cinc.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Peter Wang <peter.wang@...iatek.com>,
        Andy Gross <agross@...nel.org>,
        "Bjorn Andersson" <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>, <chu.stanley@...il.com>,
        "Alim Akhtar" <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>,
        <linux-arm-msm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <quic_cang@...cinc.com>,
        <quic_nguyenb@...cinc.com>
Subject: Re: [PATCH V2 1/3] ufs: core: Add CPU latency QoS support for ufs
 driver



On 12/7/2023 3:13 PM, Manivannan Sadhasivam wrote:
> On Wed, Dec 06, 2023 at 03:02:04PM -1000, Bart Van Assche wrote:
>> 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.
>>
> 
> Agree. The flag shouldn't be introduced if there are no users.
> 
>>> 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.
>>
> 
> Hmm, ok. Then I think we can have a sysfs hook to toggle the enable switch.

Hi Bart, Mani

How about keeping this feature enabled by default and having a module 
parameter to disable pmqos feature if required ?

Regards,
Nitin

> 
> - Mani
> 
>> Thanks,
>>
>> Bart.
>