Re: [PATCH net-next v5 1/3] net: devmem: allow rx-buf-size > PAGE_SIZE per dmabuf binding

[Jakub Kicinski <kuba@kernel.org>]

On Thu, 23 Jul 2026 16:58:14 -0700 Bobby Eshleman wrote:
> > > +	if (info->attrs[NETDEV_A_DMABUF_RX_BUF_SIZE]) {
> > > +		u32 rx_buf_size = nla_get_u32(info->attrs[NETDEV_A_DMABUF_RX_BUF_SIZE]);
> > > +
> > > +		if (!rx_buf_size || !is_power_of_2(rx_buf_size) ||
> > > +		    rx_buf_size < PAGE_SIZE) {  
> > 
> > we should add a check: min: page-size in the Netlink policy?  
> 
> I played around with this adding:
> 
> Documentation/netlink/specs/netdev.yaml:
>  definitions:
>  +  -
>  +    type: const
>  +    name: page-size
>  +    value: 4096  # dummy value, to pass ynl_gen_c.py checks
>  +    header: asm/page.h
>  +    scope: kernel
> 
> Generating:
> 
> +static const struct netlink_range_validation
> netdev_a_dmabuf_rx_page_size_range = {
> +       .min    = PAGE_SIZE,
> +       .max    = U32_MAX,
> +};
> +
> 
> ... but the dummy 4096 is kind of annoying. ynl_gen_c.py can't know the
> value of PAGE_SIZE but needs some value for its arithmetic checks (e.g.,
> confirm min < max is true).
> 
> Should we stick with using a dummy value, or should we add a patch
> teaching ynl_gen_c.py to allow value-less consts (skip the arithmetic
> checks)? 

Let's stick to a dummy one for now, but maybe something obviously dummy
like 0 ?

BTW did you add both min and max checks? Cause the only risk with using
a dummy value would be that the policy will be rendered inline, and
inline policy is u16 so 64k wouldn't fit. But your sample above has a
max of u32_max which forces the out-of-line policy, which is what we
want.