Re: [PATCH v3 14/16] arm_mpam: add MPAM-Fb MSC firmware access support

[Niyas Sait <niyas.sait@arm.com>]

On 10/07/2026 15:45, Andre Przywara wrote:

> +#define MPAM_VERSION_MSG_SIZE	(PCC_TYPE3_MSG_PAYLOAD_OFS)
> +#define MPAM_READ_MSG_SIZE	(PCC_TYPE3_MSG_PAYLOAD_OFS + 3 * sizeof(u32))
> +#define MPAM_WRITE_MSG_SIZE	(PCC_TYPE3_MSG_PAYLOAD_OFS + 4 * sizeof(u32))

I think these lengths are wrong for ACPI extended PCC shared memory.

Length should be command + payload, and should not include the payload 
offset within the PCC shared memory region.

I think this should be something like

#define MPAM_VERSION_MSG_SIZE  sizeof(u32)
#define MPAM_READ_MSG_SIZE     (sizeof(u32) + 3 * sizeof(u32))
#define MPAM_WRITE_MSG_SIZE    (sizeof(u32) + 4 * sizeof(u32))

> +
> +static int mpam_fb_build_version_message(unsigned int token,
> +					 void __iomem *msg_buf)
> +{
> +	struct acpi_pcct_ext_pcc_shared_memory *pcc_shmem = msg_buf;
> +
> +	writel_relaxed(0, &pcc_shmem->flags);

Here the flags are always 0.

If the PCCT advertises interrupt based completion, I think we need to 
set PCC_CMD_COMPLETION_NOTIFY here. Otherwise the platform can process 
the request without generating an interrupt back to the AP, and the PCC 
driver will time out waiting for completion.

> +
> +static int mpam_fb_send_request(struct mpam_pcc_chan *pcc_chan, u32 msc_id,
> +				u16 reg, u32 *result, int mpam_fb_command)
> +{
> +	unsigned int token = atomic_inc_return(&mpam_fb_token);

MPAM-Fb token is a 10-bit field in the message header, so this needs to 
be masked before using it. Otherwise once the token exceeds 1023, the 
platform returns the wrapped 10-bit token but Linux compares it against 
the original value and will probably time out.

May be something like:

unsigned int token = atomic_inc_return(&mpam_fb_token) & 
FIELD_MAX(MPAM_MSC_TOKEN_MASK);


Thanks,
Niyas