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Date:   Mon, 11 Dec 2023 15:30:02 +0530
From:   Pavan Kondeti <quic_pkondeti@quicinc.com>
To:     Manivannan Sadhasivam <mani@kernel.org>
CC:     Nitin Rawat <quic_nitirawa@quicinc.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Naresh Maramaina <quic_mnaresh@quicinc.com>,
        "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>,
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        <linux-arm-kernel@lists.infradead.org>, <quic_cang@quicinc.com>,
        <quic_nguyenb@quicinc.com>
Subject: Re: [PATCH V2 1/3] ufs: core: Add CPU latency QoS support for ufs
 driver
Message-ID: <eb0b15c5-a7e0-4144-b5f8-a8e1818b1d9f@quicinc.com>
References: <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>
 <effb603e-ca7a-4f24-9783-4d62790165ae@acm.org>
 <20231207094357.GI2932@thinkpad>
 <286b6f8a-c634-19ed-cf53-276cfe05d03f@quicinc.com>
 <20231207112101.GK2932@thinkpad>
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On Thu, Dec 07, 2023 at 04:51:01PM +0530, Manivannan Sadhasivam wrote:
> On Thu, Dec 07, 2023 at 03:56:43PM +0530, Nitin Rawat wrote:
> > 
> > 
> > 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 ?
> > 
> 
> Module params not encouraged these days unless there are no other feasible
> options available.

Yeah, one of the problem with module param is that we can't have
different settings for two two UFS controllers. Anyways, this setting
need not be a module param, there is nothing special about this setting
that is tied to module loading (driver probe) time, AFAICT.

Thanks,
Pavan