Subject: Re: [PATCH v5 net-next 6/6] net: ethernet: ti: cpsw: add XDP support

From: Ilias Apalodimas <[email-masked]>
Date: Mon, 1 Jul 2019 21:09:47 +0300

On Mon, Jul 01, 2019 at 06:19:01PM +0200, Jesper Dangaard Brouer wrote:
> On Sun, 30 Jun 2019 20:23:48 +0300
> Ivan Khoronzhuk <[email-masked]> wrote:
> 
> > +static int cpsw_ndev_create_xdp_rxq(struct cpsw_priv *priv, int ch)
> > +{
> > +	struct cpsw_common *cpsw = priv->cpsw;
> > +	int ret, new_pool = false;
> > +	struct xdp_rxq_info *rxq;
> > +
> > +	rxq = &priv->xdp_rxq[ch];
> > +
> > +	ret = xdp_rxq_info_reg(rxq, priv->ndev, ch);
> > +	if (ret)
> > +		return ret;
> > +
> > +	if (!cpsw->page_pool[ch]) {
> > +		ret =  cpsw_create_rx_pool(cpsw, ch);
> > +		if (ret)
> > +			goto err_rxq;
> > +
> > +		new_pool = true;
> > +	}
> > +
> > +	ret = xdp_rxq_info_reg_mem_model(rxq, MEM_TYPE_PAGE_POOL,
> > +					 cpsw->page_pool[ch]);
> > +	if (!ret)
> > +		return 0;
> > +
> > +	if (new_pool) {
> > +		page_pool_free(cpsw->page_pool[ch]);
> > +		cpsw->page_pool[ch] = NULL;
> > +	}
> > +
> > +err_rxq:
> > +	xdp_rxq_info_unreg(rxq);
> > +	return ret;
> > +}
> 
> Looking at this, and Ilias'es XDP-netsec error handling path, it might
> be a mistake that I removed page_pool_destroy() and instead put the
> responsibility on xdp_rxq_info_unreg().
> 
> As here, we have to detect if page_pool_create() was a success, and then
> if xdp_rxq_info_reg_mem_model() was a failure, explicitly call
> page_pool_free() because the xdp_rxq_info_unreg() call cannot "free"
> the page_pool object given it was not registered.  
> 
> Ivan's patch in[1], might be a better approach, which forced all
> drivers to explicitly call page_pool_free(), even-though it just
> dec-refcnt and the real call to page_pool_free() happened via
> xdp_rxq_info_unreg().
We did discuss that xdp_XXXXX naming might be confusing.
That being said since Ivan's approach serves 'special' hardware and fixes the
naming irregularity, i perfectly fine doing that as long as we clearly document
that the API is supposed to serve a pool per queue (unless the hardware needs to
deal with it differently)
> 
> To better handle error path, I would re-introduce page_pool_destroy(),
> as a driver API, that would gracefully handle NULL-pointer case, and
> then call page_pool_free() with the atomic_dec_and_test().  (It should
> hopefully simplify the error handling code a bit)
> 
> [1] https://lore.kernel.org/netdev/[email-masked]/
> 
>

Thanks
/Ilias
> > +void cpsw_ndev_destroy_xdp_rxqs(struct cpsw_priv *priv)
> > +{
> > +	struct cpsw_common *cpsw = priv->cpsw;
> > +	struct xdp_rxq_info *rxq;
> > +	int i;
> > +
> > +	for (i = 0; i < cpsw->rx_ch_num; i++) {
> > +		rxq = &priv->xdp_rxq[i];
> > +		if (xdp_rxq_info_is_reg(rxq))
> > +			xdp_rxq_info_unreg(rxq);
> > +	}
> > +}
> 
> Are you sure you need to test xdp_rxq_info_is_reg() here?
> 
> You should just call xdp_rxq_info_unreg(rxq), if you know that this rxq
> should be registered.  If your assumption failed, you will get a
> WARNing, and discover your driver level bug.  This is one of the ways
> the API is designed to "detect" misuse of the API.  (I found this
> rather useful, when I converted the approx 12 drivers using this
> xdp_rxq_info API).
> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer