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

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Message-ID: <1be287ac-fce9-4f27-aa88-b1f786e968cd@oss.qualcomm.com>
Date: Wed, 14 Jan 2026 11:01:21 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Georgi Djakov <djakov@...nel.org>,
        Neil Armstrong <neil.armstrong@...aro.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 1/13/26 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)

Konrad