linux-kernel - Re: [PATCH v2 2/3] iommu/arm-smmu: add ACTLR data and support for SM8550

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Date:   Thu, 16 Nov 2023 08:51:55 +0200
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Bibek Kumar Patro <quic_bibekkum@...cinc.com>
Cc:     Konrad Dybcio <konrad.dybcio@...aro.org>, will@...nel.org,
        robin.murphy@....com, joro@...tes.org, a39.skl@...il.com,
        quic_pkondeti@...cinc.com, quic_molvera@...cinc.com,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, qipl.kernel.upstream@...cinc.com
Subject: Re: [PATCH v2 2/3] iommu/arm-smmu: add ACTLR data and support for SM8550

On Thu, 16 Nov 2023 at 08:10, Bibek Kumar Patro
<quic_bibekkum@...cinc.com> wrote:
>
>
>
> On 11/15/2023 10:12 PM, Konrad Dybcio wrote:
> >
> >
> > On 11/15/23 13:49, Bibek Kumar Patro wrote:
> >>
> >>
> >> On 11/15/2023 4:15 PM, Dmitry Baryshkov wrote:
> >>> On Wed, 15 Nov 2023 at 11:51, Bibek Kumar Patro
> >>> <quic_bibekkum@...cinc.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 11/15/2023 3:08 PM, Dmitry Baryshkov wrote:
> >>>>> On Wed, 15 Nov 2023 at 11:22, Bibek Kumar Patro
> >>>>> <quic_bibekkum@...cinc.com> wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 11/14/2023 7:42 PM, Dmitry Baryshkov wrote:
> >>>>>>> On Tue, 14 Nov 2023 at 15:57, Bibek Kumar Patro
> >>>>>>> <quic_bibekkum@...cinc.com> wrote:
> >>>>>>>>
> >>>>>>>> Add ACTLR data table for SM8550 along with support for
> >>>>>>>> same including SM8550 specific implementation operations.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Bibek Kumar Patro <quic_bibekkum@...cinc.com>
> >>>>>>>> ---
> >>>>>>>>     drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 92
> >>>>>>>> +++++++++++++++++++++-
> >>>>>>>>     1 file changed, 88 insertions(+), 4 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> >>>>>>>> b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> >>>>>>>> index 578c662c7c30..0eaf6f2a2e49 100644
> >>>>>>>> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> >>>>>>>> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> >>>>>>>> @@ -25,6 +25,70 @@ struct actlr_data {
> >>>>>>>>            u32 actlr;
> >>>>>>>>     };
> >>>>>>>>
> >>>>>>>> +#define PRE_FETCH_1    0
> >>>>>>>> +#define PRE_FETCH_2    BIT(8)
> >>>>>>>> +#define PRE_FETCH_3    (BIT(9) | BIT(8))
> >>>>>>>
> >>>>>>> What is the difference between PRE_FETCH_3 and PRE_FETCH_2? And
> >>>>>>> PRE_FETCH_1? Are these real numbers that refer to some amount /
> >>>>>>> count
> >>>>>>> or just dummy names?
> >>>>>>>
> >>>>>>
> >>>>>> No,these are not real numbers, but prefetch settings for a particular
> >>>>>> perfect configuration.
> >>>>>
> >>>>> Then I'd ask for some better names or descriptions.
> >>>>>
> >>>>
> >>>> Noted, PREFETCH_SETTING_n / PREFETCH_OPTION_n sounds like a better name
> >>>> in the following case. Would it be okay to use this name instead?
> >>>
> >>> Not really.
> >>>
> >>
> >> Any suggestion you have in mind, if not this nomenclature?
> > Dmitry's concern seems to be that you provide:
> >
> > PRE_FETCH_1 /* prefetcher with settings preset no. 1 */
> > PRE_FETCH_2 /* prefetcher with settings preset no. 2 */
> > PRE_FETCH_3 /* prefetcher with settings preset no. 3 */
> >
> > whereas it would be both useful and interesting to see what these
> > settings mean, i.e. what differences there are between all of
> > these presets.
> >
>
> Ah, okay got it now from Dimitry and yours' response.
> But we exactly won't be able to reveal what each of these settings
> mean, as this might risk of revealing IP as ACTLR bits are
> implementation defined (except CPRE and CMTLB) which other SoC vendors
> might be using it in different manner(or different purpose) in their
> downstream implementation.
> We can name it like (e.g PREFETCH_DISABLE, PREFETCH_SHALLOW,
> PREFETCH_DEEP) to indicate the behaviour, but won't be exactly
> name/describe it to explain what it does with a particular setting.

This is already better than 1,2,3.

-- 
With best wishes
Dmitry