Re: [PATCH v7 09/11] arm_mpam: add MPAM-Fb MSC firmware access support

[Gavin Shan]

Hi Andre,

On 8/1/26 3:03 AM, Andre Przywara wrote:
> The Arm MPAM Firmware-backed (Fb) Profile document[1] describes an
> alternative way of accessing the "Memory System Components" (MSC) in an
> MPAM enabled system.
>
> Normally the MSCs are MMIO mapped, but in some implementations this
> might not be possible (MSC located outside of the local socket, MSC
> mapped secure-only) or desirable (direct MMIO access too slow or needs
> to be mediated through a control processor). MPAM-fb standardises a
> protocol to abstract MSC accesses, building on the SCMI protocol.
>
> Add functions that do an MSC read or write access by redirecting the
> request through a firmware interface. For now this done via an ACPI
> PCC shared memory and mailbox combination.
>
> Since the protocol used is only a small subset of the full SCMI spec,
> and the SCMI protocol has no full ACPI support anyway, open-code the
> (simple) SCMI message generation, for just the fields we need.
>
> [1] https://developer.arm.com/documentation/den0144/latest
>
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>
> Tested-by: Ritwick Sharma <ritwick.sharma@xxxxxxx>
> ---
>   drivers/resctrl/Makefile        |   2 +-
>   drivers/resctrl/mpam_devices.c  |  57 +++++++--
>   drivers/resctrl/mpam_fb.c       | 209 ++++++++++++++++++++++++++++++++
>   drivers/resctrl/mpam_internal.h |  18 +++
>   include/linux/arm_mpam.h        |   2 +-
>   5 files changed, 275 insertions(+), 13 deletions(-)
>   create mode 100644 drivers/resctrl/mpam_fb.c

[...]

> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
> new file mode 100644
> index 000000000000..61a91c6cec0a
> --- /dev/null
> +++ b/drivers/resctrl/mpam_fb.c
> @@ -0,0 +1,209 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright (C) 2024-2026 Arm Ltd.
> +
> +#include <linux/arm_mpam.h>
> +#include <linux/cleanup.h>
> +#include <linux/errno.h>
> +#include <linux/mailbox_client.h>
> +#include <linux/mutex.h>
> +#include <linux/types.h>
> +
> +#include <acpi/pcc.h>
> +#include <asm/mpam.h>
> +
> +#include "mpam_internal.h"
> +
> +#define MPAM_FB_PROTOCOL_ID		0x1a
> +
> +#define MPAM_PROTOCOL_VERSION_CMD	0x0
> +#define MPAM_MSC_ATTRIBUTES_CMD		0x3
> +#define MPAM_MSC_READ_CMD		0x4
> +#define MPAM_MSC_WRITE_CMD		0x5
> +
> +#define MPAM_FB_ERR_SUCCESS		 0
> +#define MPAM_FB_ERR_NOT_SUPPORTED	-1
> +#define MPAM_FB_ERR_INVALID_PARAMETERS	-2
> +#define MPAM_FB_ERR_DENIED		-3
> +#define MPAM_FB_ERR_NOT_FOUND		-4
> +#define MPAM_FB_ERR_OUT_OF_RANGE	-5
> +#define MPAM_FB_ERR_BUSY		-6
> +#define MPAM_FB_ERR_COMMS_ERROR		-7
> +#define MPAM_FB_ERR_GENERIC_ERROR	-8
> +#define MPAM_FB_ERR_HW_ERROR		-9
> +#define MPAM_FB_ERR_PROTOCOL_ERROR	-10
> +#define MPAM_FB_ERR_IN_USE		-11
> +
> +#define MPAM_MSC_PROT_ID_MASK		GENMASK(17, 10)
> +#define MPAM_MSC_TOKEN_MASK		GENMASK(27, 18)
> +

Why we're not using GENMASK_U32() since the operand the message is 32-bits dwords? :-)

> +#define MPAM_FB_PROT_HEADER_LEN		sizeof(u32)
> +
> +static atomic_t mpam_fb_token = ATOMIC_INIT(0);
> +
> +static void mpam_fb_build_version_message(unsigned int token,
> +					  void __iomem *msg_buf)
> +{
> +	struct acpi_pcct_ext_pcc_shared_memory __iomem *pcc_shmem = msg_buf;
> +
> +	/* .signature is filled by the platform */
> +	writel_relaxed(PCC_CMD_COMPLETION_NOTIFY, &pcc_shmem->flags);
> +	writel_relaxed(MPAM_FB_PROT_HEADER_LEN, &pcc_shmem->length);
> +	writel_relaxed(MPAM_PROTOCOL_VERSION_CMD |
> +		       FIELD_PREP(MPAM_MSC_TOKEN_MASK, token) |
> +		       FIELD_PREP(MPAM_MSC_PROT_ID_MASK, MPAM_FB_PROTOCOL_ID),
> +		       &pcc_shmem->command);
> +}

static inline void writel_relaxed(u32 value, volatile void __iomem *addr) { ...}

The input value would be 'unsigned long', which is converted to 'u32'. If  GENMASK_U32()
is used to define MPAM_MSC_TOKEN_MASK and MPAM_MSC_PROT_ID_MASK, the unexpected conversion
can be avoided.

Thanks,
Gavin