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

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Message-ID: <20231207094357.GI2932@thinkpad>
Date:   Thu, 7 Dec 2023 15:13:57 +0530
From:   Manivannan Sadhasivam <mani@...nel.org>
To:     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, Nitin Rawat <quic_nitirawa@...cinc.com>
Subject: Re: [PATCH V2 1/3] ufs: core: Add CPU latency QoS support for ufs
 driver

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.

- Mani

> Thanks,
> 
> Bart.

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