Re: [PATCH v6 10/10] arm_mpam: detect and enable MPAM-Fb PCC support

[Lee Trager <lee@trager.us>]

On 7/30/26 8:25 AM, Andre Przywara wrote:
> +static struct mpam_pcc_chan *mpam_pcc_chan_get(struct device *dev,
> +					       int subspace_id)
> +{
> +	struct mpam_pcc_chan *cur;
> +
> +	guard(mutex)(&pcc_chan_list_lock);
> +
> +	list_for_each_entry(cur, &pcc_chan_list, pcc_chans) {
> +		if (cur->subspace_id == subspace_id) {
> +			kref_get(&cur->refcount);
> +
> +			return cur;
> +		}
> +	}
> +
> +	cur = kzalloc_obj(*cur);
> +	if (!cur)
> +		return ERR_PTR(-ENOMEM);
> +
> +	cur->pcc_cl.dev = dev;
> +	cur->pcc_cl.tx_block = true;
> +
> +	cur->pcc_chan = pcc_mbox_request_channel(&cur->pcc_cl, subspace_id);
> +	if (IS_ERR(cur->pcc_chan)) {
> +		long err = PTR_ERR(cur->pcc_chan);
> +
> +		kfree(cur);
> +		return ERR_PTR(err);
> +	}
> +
> +	/* Timeout based on the "nominal latency" from the PCC ACPI table. */
> +	cur->pcc_cl.tx_tout = cur->pcc_chan->latency * 5;
I think this may need a unit conversion. ACPI 6.6 describes PCC nominal 
latency in microseconds, and the PCC mailbox driver appears to copy that 
value directly into pcc_chan->latency. However mbox_client::tx_tout is 
documented in milliseconds.

If I'm reading that correctly, assigning latency * 5 would make the 
timeout 1000 times longer than intended. Would something like this be 
better?

cur->pcc_cl.tx_tout = DIV_ROUND_UP_ULL((u64)cur->pcc_chan->latency * 5, 
USEC_PER_MSEC);