Subject: Re: [PATCH net-next 4/5] cassini: Use kmap_local_page() instead of kmap_atomic()

From: "Fabio M. De Francesco" <[email-masked]>
Date: Fri, 18 Nov 2022 09:53:42 +0100

On giovedì 17 novembre 2022 23:25:56 CET Anirudh Venkataramanan wrote:
> kmap_atomic() is being deprecated in favor of kmap_local_page().
> Replace kmap_atomic() and kunmap_atomic() with kmap_local_page() and
> kunmap_local() respectively. cas_page_map() and cas_page_unmap() aren't
> really useful anymore, so get rid of these as well.
> 
> Note that kmap_atomic() disables preemption and page-fault processing,
> but kmap_local_page() doesn't. Converting the former to the latter is safe
> only if there isn't an implicit dependency on preemption and page-fault
> handling being disabled, which does appear to be the case here.

Same NIT: again, conversions would be possible with the addition of explicit 
call(s) for disable page faults and preemption. As I said, I have no problems 
with this inaccurate description. Please see 2/5, I don't think it should 
prevent the patch to be applied.

> 
> Also note that the page being mapped is not allocated by the driver,
> and so the driver doesn't know if the page is in normal memory. This is the
> reason kmap_local_page() is used as opposed to page_address().
> 
> I don't have hardware, so this change has only been compile tested.
> 
> Cc: Ira Weiny <[email-masked]>
> Cc: Fabio M. De Francesco <[email-masked]>
> Signed-off-by: Anirudh Venkataramanan <[email-masked]>
> ---
>  drivers/net/ethernet/sun/cassini.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/sun/cassini.c
> b/drivers/net/ethernet/sun/cassini.c index 2f66cfc..3e632b0 100644
> --- a/drivers/net/ethernet/sun/cassini.c
> +++ b/drivers/net/ethernet/sun/cassini.c
> @@ -90,8 +90,6 @@
>  #include <linux/uaccess.h>
>  #include <linux/jiffies.h>
> 
> -#define cas_page_map(x)      kmap_atomic((x))
> -#define cas_page_unmap(x)    kunmap_atomic((x))
>  #define CAS_NCPUS            num_online_cpus()
> 
>  #define cas_skb_release(x)  netif_rx(x)
> @@ -2788,11 +2786,11 @@ static inline int cas_xmit_tx_ringN(struct cas *cp,
> int ring, ctrl, 0);
>  			entry = TX_DESC_NEXT(ring, entry);
> 
> -			addr = cas_page_map(skb_frag_page(fragp));
> +			addr = kmap_local_page(skb_frag_page(fragp));
>  			memcpy(tx_tiny_buf(cp, ring, entry),
>  			       addr + skb_frag_off(fragp) + len - 
tabort,
>  			       tabort);
> -			cas_page_unmap(addr);
> +			kunmap_local(addr);

memcpy_from_page() would be better suited.
Please remember to use memcpy_{from,to}_page() where they are better suited.
However, they would not change the logic, so I'm OK with this change too.

>  			mapping = tx_tiny_map(cp, ring, entry, tentry);
>  			len     = tabort;
>  		}
> --
> 2.37.2

Reviewed-by: Fabio M. De Francesco <[email-masked]>

Feel free to forward my tag, if maintainers require the use of the above-
mentioned helpers and ask for v2.

Thanks,

Fabio