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Date:   Thu, 24 Feb 2022 10:51:07 -0600
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Cc:     Alim Akhtar <alim.akhtar@samsung.com>,
        Avri Altman <avri.altman@wdc.com>,
        Rob Herring <robh+dt@kernel.org>,
        Andy Gross <agross@kernel.org>, Wei Xu <xuwei5@hisilicon.com>,
        Nishanth Menon <nm@ti.com>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Tero Kristo <kristo@kernel.org>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Jan Kotas <jank@cadence.com>, Li Wei <liwei213@huawei.com>,
        Stanley Chu <stanley.chu@mediatek.com>,
        Yaniv Gardi <ygardi@codeaurora.org>,
        linux-scsi@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org,
        linux-mediatek@lists.infradead.org
Subject: Re: [PATCH v2 09/15] scsi: ufs: deprecate 'freq-table-hz' property
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On Wed 23 Feb 03:15 CST 2022, Krzysztof Kozlowski wrote:

> On 22/02/2022 19:16, Bjorn Andersson wrote:
> > On Tue 22 Feb 06:58 PST 2022, Krzysztof Kozlowski wrote:
> > 
> >> The 'freq-table-hz' is not correct in DT schema, because '-hz' suffix
> >> defines uint32 type, not an array.  Therefore deprecate 'freq-table-hz'
> >> and use 'freq-table' instead.
> >>
> > 
> > Patch looks good in itself, but why don't we use opp-table to describe
> > the performance states?
> > 
> > In particular looking at the two columns of frequencies for various
> > Qualcomm boards they require different performance-states.
> > 
> > A concrete example is sm8350.dtsi, which specifies 75MHz and 300MHz as
> > the first frequency pair. The lower level requires the VDD_CX power rail
> > to be at least &rpmhpd_opp_low_svs, the higher frequency has a
> > required-opps of &rpmhpd_opp_nom.
> > 
> > 
> > As this isn't possible to express in the current binding we've just been
> > forced to always run at a higher voltage level and kept this in the todo
> > list.
> > 
> > But rather than migrating freq-table-hz to freq-table and then having to
> > introduce an opp table to express the power constraints, could we
> > perhaps skip the intermediate step?
> > 
> > Or would you have any other suggestion about how we can represent the
> > required-opps level together with the freq-table (if that's what we want
> > to stick with).
> 
> Usage of OPP tables is interesting solution. It would solve your problem
> of power rail levels. This would need several opp-tables - one for each
> clock, which is not a big problem.
> 

Ahh, so we can only have a single clock, but multiple regulators and
interconnect paths tied to the opp table.

We have a couple of cases where it would have been nice to be able to
key the opp-table off some index (e.g. the UFS gear or PCI Gen) and
control multiple clocks. So I think we need to look into this further...

> The problem is that I do not have any UFS hardware (none of my Samsung
> Exynos boards have UFS... I don't have even arm64 Exynos chips :( ), so
> implementing it theoretically will be painful.
> OTOH, I believe that having a working dtschema is very useful. Having
> dtschema without errors/warnings is even worth some churn/intermediary work.
> 
> The intermediary work is also not that big. Once proper OPP is
> implemented, we will have "just" two deprecated properties in the bindings.
> 

Fair enough, was just hoping to avoid the middle step. But that's fine,
we'll continue to carry this on our todo list then.

Thanks,
Bjorn