linux-kernel - RE: [PATCH 3/8] scsi: ufshpb: Add region's reads counter

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Message-ID: <DM6PR04MB65751A2F42801BAACF073F6FFCB69@DM6PR04MB6575.namprd04.prod.outlook.com>
Date:   Mon, 1 Feb 2021 08:17:59 +0000
From:   Avri Altman <Avri.Altman@....com>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "daejun7.park@...sung.com" <daejun7.park@...sung.com>,
        "James E . J . Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bart Van Assche <bvanassche@....org>,
        yongmyung lee <ymhungry.lee@...sung.com>,
        ALIM AKHTAR <alim.akhtar@...sung.com>,
        "asutoshd@...eaurora.org" <asutoshd@...eaurora.org>,
        Zang Leigang <zangleigang@...ilicon.com>,
        Avi Shchislowski <Avi.Shchislowski@....com>,
        Bean Huo <beanhuo@...ron.com>,
        "cang@...eaurora.org" <cang@...eaurora.org>,
        "stanley.chu@...iatek.com" <stanley.chu@...iatek.com>
Subject: RE: [PATCH 3/8] scsi: ufshpb: Add region's reads counter

> 
> On Mon, Feb 01, 2021 at 07:51:19AM +0000, Avri Altman wrote:
> > >
> > > On Mon, Feb 01, 2021 at 07:12:53AM +0000, Avri Altman wrote:
> > > > > > +#define WORK_PENDING 0
> > > > > > +#define ACTIVATION_THRSHLD 4 /* 4 IOs */
> > > > > Rather than fixing it with macro, how about using sysfs and make it
> > > > > configurable?
> > > > Yes.
> > > > I will add a patch making all the logic configurable.
> > > > As all those are hpb-related parameters, I think module parameters are
> > > more adequate.
> > >
> > > No, this is not the 1990's, please never add new module parameters to
> > > drivers.  If not for the basic problem of they do not work on a
> > > per-device basis, but on a per-driver basis, which is what you almost
> > > never want.
> > OK.
> >
> > >
> > > But why would you want to change this value, why can't the driver "just
> > > work" and not need manual intervention?
> > It is.
> > But those are a knobs each vendor may want to tweak,
> > So it'll be optimized with its internal device's implementation.
> >
> > Tweaking the parameters, as well as the entire logic, is really an endless
> task.
> > Some logic works better for some scenarios, while falling behind on others.
> 
> Shouldn't the hardware know how to handle this dynamically?  If not, how
> is a user going to know?
There is one "brain".
It is either in the device - in device mode, Or in the host - in host mode control.
The "brain" decides which region is active, thus carrying the physical address along with the logical -
minimizing context switches in the device's RAM.

There can be up to N active regions.
Activation and deactivation has its overhead.
So basically it is a constraint-optimization problem.

> 
> > How about leaving it for now, to be elaborated it in the future?
> 
> I do not care, just do not make it a module parameter for the reason
> that does not work on a per-device basis.
OK.  Will make it a sysfs per hpb-lun, like Daejun proposed.

Thanks,
Avri