linux-kernel - Re: [PATCH][next] optee: make read-only array attr static const

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Message-ID: <20260117141031.1514faf2@pumpkin>
Date: Sat, 17 Jan 2026 14:10:31 +0000
From: David Laight <david.laight.linux@...il.com>
To: Colin Ian King <coking@...dia.com>
Cc: Jens Wiklander <jens.wiklander@...aro.org>, Sumit Garg
 <sumit.garg@...nel.org>, op-tee@...ts.trustedfirmware.org,
 kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] optee: make read-only array attr static const

On Fri,  9 Jan 2026 15:44:42 +0000
Colin Ian King <coking@...dia.com> wrote:

> Don't populate the read-only array attr on the stack at run
> time, instead make it static const.

For a 4 byte array initialising on stack is likely to generate faster code.
In particular it avoid a very likely data-cache miss for the rodata.
The on-stack array access might even shorter code than accessing a global
array on some architectures.
The compiler might, of course, decide to unroll the loop and not instantiate
the array at all (in either case).
Indeed, just unrolling the loop as:
	if (params[0].attr != TEE_IOCTL_PARAM_ATTR_TYPE_VALUE_INPUT) goto bad;
	if (params[1].attr != TEE_IOCTL_PARAM_ATTR_TYPE_VALUE_INPUT) goto bad;
	if (params[2].attr != TEE_IOCTL_PARAM_ATTR_TYPE_MEMREF_INOUT) goto bad;
	if (params[3].attr != TEE_IOCTL_PARAM_ATTR_TYPE_VALUE_OUTPUT) goto bad;
is likely to be pretty near optimal.
(Collecting the bytes and doing a single 32bit compare may be better.)

Of course this won't be a massively hot path - so the code choice won't
make a measurable difference.

	David


> 
> Signed-off-by: Colin Ian King <coking@...dia.com>
> ---
>  drivers/tee/optee/rpc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tee/optee/rpc.c b/drivers/tee/optee/rpc.c
> index 97fc5b14db0c..1758eb7e6e8b 100644
> --- a/drivers/tee/optee/rpc.c
> +++ b/drivers/tee/optee/rpc.c
> @@ -43,7 +43,7 @@ static void handle_rpc_func_cmd_i2c_transfer(struct tee_context *ctx,
>  	struct i2c_msg msg = { };
>  	size_t i;
>  	int ret = -EOPNOTSUPP;
> -	u8 attr[] = {
> +	static const u8 attr[] = {
>  		TEE_IOCTL_PARAM_ATTR_TYPE_VALUE_INPUT,
>  		TEE_IOCTL_PARAM_ATTR_TYPE_VALUE_INPUT,
>  		TEE_IOCTL_PARAM_ATTR_TYPE_MEMREF_INOUT,