linux-kernel - Re: [PATCH 2/3] firmware: arm_scmi: Fix freq/power truncation in the perf protocol

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Message-ID: <c8e1adbf-a63e-5a6f-9b4a-e68a2af67779@quicinc.com>
Date:   Fri, 1 Dec 2023 01:02:25 +0530
From:   Sibi Sankar <quic_sibis@...cinc.com>
To:     Cristian Marussi <cristian.marussi@....com>,
        Sudeep Holla <sudeep.holla@....com>
CC:     <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <quic_mdtipton@...cinc.com>,
        <linux-arm-kernel@...ts.infradead.org>, <quic_asartor@...cinc.com>,
        <quic_lingutla@...cinc.com>
Subject: Re: [PATCH 2/3] firmware: arm_scmi: Fix freq/power truncation in the
 perf protocol



On 11/30/23 21:55, Cristian Marussi wrote:
> On Thu, Nov 30, 2023 at 01:56:56PM +0000, Sudeep Holla wrote:
>> On Thu, Nov 30, 2023 at 12:49:42PM +0000, Cristian Marussi wrote:
>>> On Thu, Nov 30, 2023 at 12:05:06PM +0000, Sudeep Holla wrote:
>>>> On Wed, Nov 29, 2023 at 12:27:47PM +0530, Sibi Sankar wrote:
>>>>> Fix frequency and power truncation seen in the performance protocol by
>>>>> casting it with the correct type.
>>>>>
>>>>
>>>> While I always remembered to handle this when reviewing the spec, seem to
>>>> have forgotten when it came to handling in the implementation :(. Thanks
>>>> for spotting this.
>>>>
>>>> However I don't like the ugly type casting. I think we can do better. Also
>>>> looking at the code around the recently added level index mode, I think we
>>>> can simplify things like below patch.
>>>>
>>>> Cristian,
>>>> What do you think ?
>>>>
>>>
>>> Hi
>>>
>>> the cleanup seems nice in general to compact the mult_factor multipliers
>>> in one place, and regarding addressing the problem of truncation without
>>> the need of the explicit casting, should not be enough to change to
>>> additionally also change mult_factor to be an u64 ?
>>>
>>
>> I started exactly with that, but when I completed the patch, there was no
>> explicit need for it, so dropped it again. I can bump mult_factor to be
>> u64 but do you see any other place that would need it apart from having
>> single statement that does multiplication and assignment ? I am exploiting
>> the conditional based on level_indexing_mode here but I agree it may help
>> in backporting if I make mult_factor u64.
>>
> 
> Ah right
> 
>     freq *= dom->multi_fact;
> 
> does the trick..but cannot this by itself (under unplausibl conds)
> overflow and does not fit into a u32 mult_factor ?
> 
>   dom_info->mult_factor =
>   	(dom_info->sustained_freq_khz * 1000UL)

wouldn't having the 1000UL ensure that we don't truncate though?
Anyway will drop the patch when I re-spin the series.

-Sibi

> 	/ dom_info->sustained_perf_level;
> 
> 
> Thanks,
> Cristian
>