linux-kernel - Re: [PATCH RFC RFT] interconnect: qcom: implement get_bw with rpmh

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Message-ID: <95becfde-ba4b-4024-9b90-e64e77551f0a@linaro.org>
Date: Wed, 14 Jan 2026 11:07:37 +0100
From: Neil Armstrong <neil.armstrong@...aro.org>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
 Georgi Djakov <djakov@...nel.org>
Cc: linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 Bjorn Andersson <bjorn.andersson@....qualcomm.com>
Subject: Re: [PATCH RFC RFT] interconnect: qcom: implement get_bw with
 rpmh_read

On 1/14/26 11:01, Konrad Dybcio wrote:
> On 1/13/26 6:53 PM, Georgi Djakov wrote:
>> On 11/6/25 6:46 PM, Neil Armstrong wrote:
>>> Since we can actually read back the APPS rpmh interconnect
>>> BCM votes we can actually implement the get_bw() callback
>>> and provide a coherent average and peak bandwidth at probe time.
>>>
>>> The benefits of that are:
>>> - keep disabled BCMs disabled
>>> - avoid voting unused BCMs to INT_MAX
>>>
>>> If the interconnects are correctly described for a platform,
>>> all the required BCMs would be voted to the maximum bandwidth
>>> until sync_state is reached.
>>>
>>> Since we only get the BCM vote, we need to redistribute
>>> the vote values to the associated nodes. The initial BCM
>>> votes are read back at probe time in order to be ready when
>>> the get_bw() is called when a node is added.
>>>
>>
>> FWIW, I was able to finally test this on sdm845. Some nodes are indeed
>> showing reasonable bandwidth values instead of the default INT_MAX.
> 
> As I learnt here
> 
> https://lore.kernel.org/linux-arm-msm/1e7594dc-dca6-42e7-b478-b063e3325aff@oss.qualcomm.com/
> 
> rpmh_read() will only retrieve the currently active values, so as-is,
> this hunk:
> 
> +	/* For boot-up, fill the AMC vote in all buckets */
> +	for (i = 0; i < QCOM_ICC_NUM_BUCKETS; i++) {
> +		bcm->vote_x[i] = x;
> +		bcm->vote_y[i] = y;
> +	}
> 
> is lying about the state of wake/sleep buckets
> 
> this is ""fine"" today, as I don't see any "if (old_bw == new_bw)" checks
> across the framework, but debugfs is going to report incorrect values and
> if anyone decides to add the aforementioned check, it may introduce issues
> where the values aren't commited to the hardware (because Linux is going
> to believe they're already set)

This is only for the pre-sync-state phase, where we don't need the wake/sleep
values but the interconnect rpmh implementation needs them, and anyway they will
be replaced by proper values in sync_state

So this is an informed & assumed choice I did here. It's a small optimization
to avoid turning on _all_ interconnects at INT_MAX, and keep boot votes
up to sync_state.

Neil

> 
> Konrad