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

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Date:   Fri, 17 Nov 2023 11:06:26 +0530
From:   Bibek Kumar Patro <quic_bibekkum@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
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 11/16/2023 12:21 PM, Dmitry Baryshkov wrote:
> 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.
> 

Acked, will take care of this in next version.