Subject: Re: [PATCH 1/2] firmware: arm_ffa: Honor maximum RX/TX buffer size

From: Sudeep Holla <[email-masked]>
Date: Tue, 2 Jun 2026 16:56:48 +0100

On Mon, Jun 01, 2026 at 03:45:11PM -0500, Seth Forshee wrote:
> FFA_FEATURES in v1.2+ supports a maximum RXTX_MAP size. This maximum
> size is not checked when page-aligning the RX/TX buffer size, and
> FFA_RXTX_MAP may fail when passed a size greater than the maximum.
> 
> Decode the maximum buffer size returned from FFA_FEATURES and limit the
> buffer size based on this value if it is non-zero (zero indicates no
> maximum). Include verification that the max returned by the SPMC is
> larger than the minimum, otherwise use the minimum.
> 
> While there, also update RXTX_MAP_MIN_BUFSZ() to use FIELD_GET() for
> consistency.
> 
> Fixes: 83210251fd70 ("firmware: arm_ffa: Use the correct buffer size during RXTX_MAP")
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Seth Forshee <[email-masked]>
> ---
>  drivers/firmware/arm_ffa/driver.c | 29 +++++++++++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/firmware/arm_ffa/driver.c b/drivers/firmware/arm_ffa/driver.c
> index b9f17fda7243..dc45724a29ba 100644
> --- a/drivers/firmware/arm_ffa/driver.c
> +++ b/drivers/firmware/arm_ffa/driver.c
> @@ -32,6 +32,7 @@
>  #include <linux/interrupt.h>
>  #include <linux/io.h>
>  #include <linux/kernel.h>
> +#include <linux/minmax.h>
>  #include <linux/module.h>
>  #include <linux/mm.h>
>  #include <linux/mutex.h>
> @@ -55,7 +56,9 @@
>  	(FIELD_PREP(SENDER_ID_MASK, (s)) | FIELD_PREP(RECEIVER_ID_MASK, (r)))
>  
>  #define RXTX_MAP_MIN_BUFSZ_MASK	GENMASK(1, 0)
> -#define RXTX_MAP_MIN_BUFSZ(x)	((x) & RXTX_MAP_MIN_BUFSZ_MASK)
> +#define RXTX_MAP_MAX_BUFSZ_MASK	GENMASK(31, 16)
> +#define RXTX_MAP_MIN_BUFSZ(x)	(FIELD_GET(RXTX_MAP_MIN_BUFSZ_MASK, (x)))
> +#define RXTX_MAP_MAX_BUFSZ(x)	(FIELD_GET(RXTX_MAP_MAX_BUFSZ_MASK, (x)))
>  
>  #define FFA_MAX_NOTIFICATIONS		64
>  
> @@ -2086,11 +2089,13 @@ static void ffa_notifications_setup(void)
>  	ffa_notifications_cleanup();
>  }
>  
> +#define FFA_SUPPORTS_RXTX_MAX_BUFSZ(version)	((version) > FFA_VERSION_1_1)
> +
>  static int __init ffa_init(void)
>  {
>  	int ret;
>  	u32 buf_sz;
> -	size_t rxtx_bufsz = SZ_4K;
> +	size_t rxtx_bufsz = SZ_4K, rxtx_max_bufsz = 0;
>  
>  	ret = ffa_transport_init(&invoke_ffa_fn);
>  	if (ret)
> @@ -2118,9 +2123,29 @@ static int __init ffa_init(void)
>  			rxtx_bufsz = SZ_16K;
>  		else
>  			rxtx_bufsz = SZ_4K;
> +
> +		if (FFA_SUPPORTS_RXTX_MAX_BUFSZ(drv_info->version)) {

This is not even compile tested ? I don't think this is defined anyway.
Anyways I don't think we need this condition as v1.1 or below had it
MBZ, so it is fine to drop it.

> +			rxtx_max_bufsz = (size_t)RXTX_MAP_MAX_BUFSZ(buf_sz) * SZ_4K;

The cast is unnecessary, its is 16 bit, so max 0xFFFF << 12.

> +			if (rxtx_max_bufsz != 0 && rxtx_max_bufsz < rxtx_bufsz) {
> +				/*
> +				 * Per spec the maximum must be >= the minimum, or
> +				 * else zero if there is no size limit. If the SPMC
> +				 * violates this constraint, use the minimum as the
> +				 * effective maximum.
> +				 */

I don't see any text implying the above from the spec, drop it. Please point
it to me if you find. Even otherwise it seems obvious here and doesn't need
the note.

-- 
Regards,
Sudeep