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

[Bobby Eshleman]

On Tue, Jul 21, 2026 at 11:07:13AM -0700, Jakub Kicinski wrote:
> On Wed, 08 Jul 2026 15:55:04 -0700 Bobby Eshleman wrote:
> > diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
> > index 5f143da7458c..70b902008bd3 100644
> > --- a/Documentation/netlink/specs/netdev.yaml
> > +++ b/Documentation/netlink/specs/netdev.yaml
> > @@ -598,6 +598,13 @@ attribute-sets:
> >          type: u32
> >          checks:
> >            min: 1
> > +      -
> > +        name: rx-buf-size
> 
> rx-page-size, we're modeling device pages.
> rx-buf-len exists in ethtool and is something else.
> 
> > +	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)? 

Best,
Bobby

> 
> > +			NL_SET_ERR_MSG_FMT(info->extack,
> > +					   "rx_buf_size %u must be a power of 2 >= page size (%lu)",
> > +					   rx_buf_size, PAGE_SIZE);
> 
> This must point at _ATTR, the _FMT is unnecessary.
> -- 
> pw-bot: cr