linux-kernel - Re: [PATCH 10/33] arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate

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Message-ID: <d4aa5d0d-da66-4350-a9e1-ce7fb39f2961@arm.com>
Date: Fri, 5 Sep 2025 19:52:36 +0100
From: James Morse <james.morse@....com>
To: Rob Herring <robh@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-acpi@...r.kernel.org, devicetree@...r.kernel.org,
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 Will Deacon <will@...nel.org>,
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 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH 10/33] arm_mpam: Add probe/remove for mpam msc driver and
 kbuild boiler plate

Hi Rob,

On 27/08/2025 17:16, Rob Herring wrote:
> On Wed, Aug 27, 2025 at 10:39 AM Rob Herring <robh@...nel.org> wrote:
>>
>> On Fri, Aug 22, 2025 at 10:32 AM James Morse <james.morse@....com> wrote:
>>>
>>> Probing MPAM is convoluted. MSCs that are integrated with a CPU may
>>> only be accessible from those CPUs, and they may not be online.
>>> Touching the hardware early is pointless as MPAM can't be used until
>>> the system-wide common values for num_partid and num_pmg have been
>>> discovered.
>
> [...]
>
>>> +static int mpam_dt_parse_resources(struct mpam_msc *msc, void *ignored)
>>> +{
>>> +       int err, num_ris = 0;
>>> +       const u32 *ris_idx_p;
>>> +       struct device_node *iter, *np;
>>> +
>>> +       np = msc->pdev->dev.of_node;
>>> +       for_each_child_of_node(np, iter) {
>>
>> Use for_each_available_child_of_node_scoped()
>>
>>> +               ris_idx_p = of_get_property(iter, "reg", NULL);
>>
>> This is broken on big endian and new users of of_get_property() are
>> discouraged. Use of_property_read_reg().
>
> Err, this is broken on little endian as the DT is big endian.
>
> So this was obviously not tested as I'm confident you didn't test on BE.

'not tested' is shades of grey. I fed the FVP ~6 different DTB files to hit the different
paths through the driver. The FVP only has controls under RIS-0, so all of those only
defined RIS-0, and unsurprisingly didn't notice the helper isn't endian safe.


Thanks,

James