linux-kernel - Re: [PATCH 20/20] interconnect: qcom: Divide clk rate by src node bus width

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Date:   Thu, 1 Jun 2023 15:23:55 +0200
From:   Stephan Gerhold <stephan@...hold.net>
To:     Konrad Dybcio <konrad.dybcio@...aro.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Georgi Djakov <djakov@...nel.org>,
        Leo Yan <leo.yan@...aro.org>,
        Evan Green <evgreen@...omium.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 20/20] interconnect: qcom: Divide clk rate by src node
 bus width

On Thu, Jun 01, 2023 at 02:43:50PM +0200, Konrad Dybcio wrote:
> On 30.05.2023 21:02, Stephan Gerhold wrote:
> > On Tue, May 30, 2023 at 06:32:04PM +0200, Konrad Dybcio wrote:
> >> On 30.05.2023 12:20, Konrad Dybcio wrote:
> >>> Ever since the introduction of SMD RPM ICC, we've been dividing the
> >>> clock rate by the wrong bus width. This has resulted in:
> >>>
> >>> - setting wrong (mostly too low) rates, affecting performance
> >>>   - most often /2 or /4
> >>>   - things like DDR never hit their full potential
> >>>   - the rates were only correct if src bus width == dst bus width
> >>>     for all src, dst pairs on a given bus
> >>>
> >>> - Qualcomm using the same wrong logic in their BSP driver in msm-5.x
> >>>   that ships in production devices today
> >>>
> >>> - me losing my sanity trying to find this
> >>>
> >>> Resolve it by using dst_qn, if it exists.
> >>>
> >>> Fixes: 5e4e6c4d3ae0 ("interconnect: qcom: Add QCS404 interconnect provider driver")
> >>> Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
> >>> ---
> >> The problem is deeper.
> >>
> >> Chatting with Stephan (+CC), we tackled a few issues (that I will send
> >> fixes for in v2):
> >>
> >> 1. qcom_icc_rpm_set() should take per-node (src_qn->sum_avg, dst_qn->sum_avg)
> >>    and NOT aggregated bw (unless you want ALL of your nodes on a given provider
> >>    to "go very fast")
> >>
> >> 2. the aggregate bw/clk rate calculation should use the node-specific bus widths
> >>    and not only the bus width of the src/dst node, otherwise the average bw
> >>    values will be utterly meaningless
> >>
> > 
> > The peak bandwidth / clock rate is wrong as well if you have two paths
> > with different buswidths on the same bus/NoC. (If someone is interested
> > in details I can post my specific example I had in the chat, it shows
> > this more clearly.)
> agg_peak takes care of that, I believe..
> 

I was just nitpicking on your description here, I think the solution
you/we had in mind was already correct. :)

> 
> > 
> >> 3. thanks to (1) and (2) qcom_icc_bus_aggregate() can be remodeled to instead
> >>    calculate the clock rates for the two rpm contexts, which we can then max()
> >>    and pass on to the ratesetting call
> >>
> > 
> > Sounds good.
> > 
> >>
> >> ----8<---- Cutting off Stephan's seal of approval, this is my thinking ----
> >>
> >> 4. I *think* Qualcomm really made a mistake in their msm-5.4 driver where they
> >>    took most of the logic from the current -next state and should have been
> >>    setting the rate based on the *DST* provider, or at least that's my
> >>    understanding trying to read the "known good" msm-4.19 driver
> >>    (which remembers msm-3.0 lol).. Or maybe we should keep src but ensure there's
> >>    also a final (dst, dst) vote cast:
> >>
> >> provider->inter_set = false // current state upstream
> >>
> >> setting apps_proc<->slv_bimc_snoc
> >> setting mas_bimc_snoc<->slv_snoc_cnoc
> >> setting mas_snoc_cnoc<->qhs_sdc2
> >>
> >>
> >> provider->inter_set = true // I don't think there's effectively a difference?
> >>
> >> setting apps_proc<->slv_bimc_snoc
> >> setting slv_bimc_snoc<->mas_bimc_snoc
> >> setting mas_bimc_snoc<->slv_snoc_cnoc
> >> setting slv_snoc_cnoc<->mas_snoc_cnoc
> >> setting mas_snoc_cnoc<->qhs_sdc2
> >>
> > 
> > I think with our proposed changes above it does no longer matter if a
> > node is passed as "src" or "dst". This means in your example above you
> > just waste additional time setting the bandwidth twice for
> > slv_bimc_snoc, mas_bimc_snoc, slv_snoc_cnoc and mas_snoc_cnoc.
> > The final outcome is the same with or without "inter_set".
> Yeah I guess due to the fact that two "real" nodes are always
> connected by a set of "gateway" nodes, the rate will be applied..
> 
> I am however not sure if we're supposed to set the bandwidth
> (via qcom_icc_rpm_set()) on all of them..
> 

I think so? The nodes RPM doesn't care about shouldn't have
a slv/mas_rpm_id.