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

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

[Colin Ian King via OP-TEE]

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

Signed-off-by: Colin Ian King <coking@nvidia.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,
-- 
2.51.0

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

[Sumit Garg via OP-TEE]

On Fri, Jan 09, 2026 at 03:44:42PM +0000, Colin Ian King wrote:
> Don't populate the read-only array attr on the stack at run
> time, instead make it static const.

Is there any value add to do this? AFAIK, the static local variables
aren't preffered.

-Sumit

> 
> Signed-off-by: Colin Ian King <coking@nvidia.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,
> -- 
> 2.51.0
> 

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

[ryan foster]

This looks like a micro-optimization, const makes the lookup array
explicitly immutable, and static keeps it out of the stack frame,
avoiding per-call initialization.

Is there a style preference for read only lookup arrays here, e.g.
Should these variables remain local but not static, or should they be
moved to file scope static const?

On Mon, Jan 12, 2026 at 3:26 AM Sumit Garg <sumit.garg@kernel.org> wrote:
>
> On Fri, Jan 09, 2026 at 03:44:42PM +0000, Colin Ian King wrote:
> > Don't populate the read-only array attr on the stack at run
> > time, instead make it static const.
>
> Is there any value add to do this? AFAIK, the static local variables
> aren't preffered.
>
> -Sumit
>
> >
> > Signed-off-by: Colin Ian King <coking@nvidia.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,
> > --
> > 2.51.0
> >
>

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

[Colin King via OP-TEE]

I added this change for several reasons, it's read-only so making it const is always sensible.  Making it static means we don't get the compiler to generate object code that stores the array on the stack (each time it's called) and then index into it, instead the data is put into a read-only data section at compile timne and the object code just fetched data from this array.

Basically, if it's read-only it's good to make it const and it's not in an __init section then you may as well make it static const. It's scoped inside the function, even though it's in the data section.


________________________________
From: ryan foster <foster.ryan.r@gmail.com>
Sent: 12 January 2026 14:46
To: Sumit Garg <sumit.garg@kernel.org>
Cc: Colin King <coking@nvidia.com>; Jens Wiklander <jens.wiklander@linaro.org>; op-tee@lists.trustedfirmware.org <op-tee@lists.trustedfirmware.org>; kernel-janitors@vger.kernel.org <kernel-janitors@vger.kernel.org>; linux-kernel@vger.kernel.org <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH][next] optee: make read-only array attr static const

This looks like a micro-optimization, const makes the lookup array
explicitly immutable, and static keeps it out of the stack frame,
avoiding per-call initialization.

Is there a style preference for read only lookup arrays here, e.g.
Should these variables remain local but not static, or should they be
moved to file scope static const?

On Mon, Jan 12, 2026 at 3:26 AM Sumit Garg <sumit.garg@kernel.org> wrote:
>
> On Fri, Jan 09, 2026 at 03:44:42PM +0000, Colin Ian King wrote:
> > Don't populate the read-only array attr on the stack at run
> > time, instead make it static const.
>
> Is there any value add to do this? AFAIK, the static local variables
> aren't preffered.
>
> -Sumit
>
> >
> > Signed-off-by: Colin Ian King <coking@nvidia.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,
> > --
> > 2.51.0
> >
>

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

[Sumit Garg via OP-TEE]

On Mon, Jan 12, 2026 at 03:24:49PM +0000, Colin King wrote:
> I added this change for several reasons, it's read-only so making it const is always sensible.  Making it static means we don't get the compiler to generate object code that stores the array on the stack (each time it's called) and then index into it, instead the data is put into a read-only data section at compile timne and the object code just fetched data from this array.
> 
> Basically, if it's read-only it's good to make it const and it's not in an __init section then you may as well make it static const. It's scoped inside the function, even though it's in the data section.
> 

That's fair. I don't have a strict preference here. Feel free to add:

Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>

-Sumit

> 
> ________________________________
> From: ryan foster <foster.ryan.r@gmail.com>
> Sent: 12 January 2026 14:46
> To: Sumit Garg <sumit.garg@kernel.org>
> Cc: Colin King <coking@nvidia.com>; Jens Wiklander <jens.wiklander@linaro.org>; op-tee@lists.trustedfirmware.org <op-tee@lists.trustedfirmware.org>; kernel-janitors@vger.kernel.org <kernel-janitors@vger.kernel.org>; linux-kernel@vger.kernel.org <linux-kernel@vger.kernel.org>
> Subject: Re: [PATCH][next] optee: make read-only array attr static const
> 
> This looks like a micro-optimization, const makes the lookup array
> explicitly immutable, and static keeps it out of the stack frame,
> avoiding per-call initialization.
> 
> Is there a style preference for read only lookup arrays here, e.g.
> Should these variables remain local but not static, or should they be
> moved to file scope static const?
> 
> On Mon, Jan 12, 2026 at 3:26 AM Sumit Garg <sumit.garg@kernel.org> wrote:
> >
> > On Fri, Jan 09, 2026 at 03:44:42PM +0000, Colin Ian King wrote:
> > > Don't populate the read-only array attr on the stack at run
> > > time, instead make it static const.
> >
> > Is there any value add to do this? AFAIK, the static local variables
> > aren't preffered.
> >
> > -Sumit
> >
> > >
> > > Signed-off-by: Colin Ian King <coking@nvidia.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,
> > > --
> > > 2.51.0
> > >
> >

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

[Jens Wiklander]

On Fri, Jan 16, 2026 at 10:14 AM Sumit Garg <sumit.garg@kernel.org> wrote:
>
> On Mon, Jan 12, 2026 at 03:24:49PM +0000, Colin King wrote:
> > I added this change for several reasons, it's read-only so making it const is always sensible.  Making it static means we don't get the compiler to generate object code that stores the array on the stack (each time it's called) and then index into it, instead the data is put into a read-only data section at compile timne and the object code just fetched data from this array.
> >
> > Basically, if it's read-only it's good to make it const and it's not in an __init section then you may as well make it static const. It's scoped inside the function, even though it's in the data section.
> >
>
> That's fair. I don't have a strict preference here. Feel free to add:
>
> Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>

I'm picking up this.

Thanks,
Jens

>
> -Sumit
>
> >
> > ________________________________
> > From: ryan foster <foster.ryan.r@gmail.com>
> > Sent: 12 January 2026 14:46
> > To: Sumit Garg <sumit.garg@kernel.org>
> > Cc: Colin King <coking@nvidia.com>; Jens Wiklander <jens.wiklander@linaro.org>; op-tee@lists.trustedfirmware.org <op-tee@lists.trustedfirmware.org>; kernel-janitors@vger.kernel.org <kernel-janitors@vger.kernel.org>; linux-kernel@vger.kernel.org <linux-kernel@vger.kernel.org>
> > Subject: Re: [PATCH][next] optee: make read-only array attr static const
> >
> > This looks like a micro-optimization, const makes the lookup array
> > explicitly immutable, and static keeps it out of the stack frame,
> > avoiding per-call initialization.
> >
> > Is there a style preference for read only lookup arrays here, e.g.
> > Should these variables remain local but not static, or should they be
> > moved to file scope static const?
> >
> > On Mon, Jan 12, 2026 at 3:26 AM Sumit Garg <sumit.garg@kernel.org> wrote:
> > >
> > > On Fri, Jan 09, 2026 at 03:44:42PM +0000, Colin Ian King wrote:
> > > > Don't populate the read-only array attr on the stack at run
> > > > time, instead make it static const.
> > >
> > > Is there any value add to do this? AFAIK, the static local variables
> > > aren't preffered.
> > >
> > > -Sumit
> > >
> > > >
> > > > Signed-off-by: Colin Ian King <coking@nvidia.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,
> > > > --
> > > > 2.51.0
> > > >
> > >

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

[Dan Carpenter]

On Mon, Jan 12, 2026 at 04:56:10PM +0530, Sumit Garg wrote:
> On Fri, Jan 09, 2026 at 03:44:42PM +0000, Colin Ian King wrote:
> > Don't populate the read-only array attr on the stack at run
> > time, instead make it static const.
> 
> Is there any value add to do this? AFAIK, the static local variables
> aren't preffered.
> 

I have never heard that static local variables aren't prefered.  I
imagine that because they're often not thread safe?  But if they're
const thread safeness isn't an problem.  Generally, we prefer the
scope as local as possible.

regards,
dan carpenter

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

[David Laight]

On Fri,  9 Jan 2026 15:44:42 +0000
Colin Ian King <coking@nvidia.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@nvidia.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,