libeth uses netmems for quite some time already, so in order to
support unreadable frags / memory providers, it only needs to set
PP_FLAG_ALLOW_UNREADABLE_NETMEM when needed.
Also add a couple sanity checks to make sure the driver didn't mess
up the configuration options and, in case when an MP is installed,
return the truesize always equal to PAGE_SIZE, so that
libeth_rx_alloc() will never try to allocate frags. Memory providers
manage buffers on their own and expect 1:1 buffer / HW Rx descriptor
association.

Bonus: mention in the libeth_sqe_type description that
LIBETH_SQE_EMPTY should also be used for netmem Tx SQEs -- they
don't need DMA unmapping.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
---
 include/net/libeth/tx.h                |  2 +-
 drivers/net/ethernet/intel/libeth/rx.c | 45 ++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/include/net/libeth/tx.h b/include/net/libeth/tx.h
index c3db5c6f1641..a66fc2b3a114 100644
--- a/include/net/libeth/tx.h
+++ b/include/net/libeth/tx.h
@@ -12,7 +12,7 @@
 
 /**
  * enum libeth_sqe_type - type of &libeth_sqe to act on Tx completion
- * @LIBETH_SQE_EMPTY: unused/empty OR XDP_TX/XSk frame, no action required
+ * @LIBETH_SQE_EMPTY: empty OR netmem/XDP_TX/XSk frame, no action required
  * @LIBETH_SQE_CTX: context descriptor with empty SQE, no action required
  * @LIBETH_SQE_SLAB: kmalloc-allocated buffer, unmap and kfree()
  * @LIBETH_SQE_FRAG: mapped skb frag, only unmap DMA
diff --git a/drivers/net/ethernet/intel/libeth/rx.c b/drivers/net/ethernet/intel/libeth/rx.c
index 9ac3a1448b2f..9b45c9cdd599 100644
--- a/drivers/net/ethernet/intel/libeth/rx.c
+++ b/drivers/net/ethernet/intel/libeth/rx.c
@@ -6,6 +6,7 @@
 #include <linux/export.h>
 
 #include <net/libeth/rx.h>
+#include <net/netdev_queues.h>
 
 /* Rx buffer management */
 
@@ -139,9 +140,50 @@ static bool libeth_rx_page_pool_params_zc(struct libeth_fq *fq,
 	fq->buf_len = clamp(mtu, LIBETH_RX_BUF_STRIDE, max);
 	fq->truesize = fq->buf_len;
 
+	/*
+	 * Allow frags only for kernel pages. `fq->truesize == pp->max_len`
+	 * will always fall back to regular page_pool_alloc_netmems()
+	 * regardless of the MTU / FQ buffer size.
+	 */
+	if (pp->flags & PP_FLAG_ALLOW_UNREADABLE_NETMEM)
+		fq->truesize = pp->max_len;
+
 	return true;
 }
 
+/**
+ * libeth_rx_page_pool_check_unread - check input params for unreadable MPs
+ * @fq: buffer queue to check
+ * @pp: &page_pool_params for the queue
+ *
+ * Make sure we don't create an invalid pool with full-frame unreadable
+ * buffers, bidirectional unreadable buffers or so, and configure the
+ * ZC payload pool accordingly.
+ *
+ * Return: true on success, false on invalid input params.
+ */
+static bool libeth_rx_page_pool_check_unread(const struct libeth_fq *fq,
+					     struct page_pool_params *pp)
+{
+	if (!pp->netdev)
+		return true;
+
+	if (!netif_rxq_has_unreadable_mp(pp->netdev, pp->queue_idx))
+		return true;
+
+	/* For now, the core stack doesn't allow XDP with unreadable frags */
+	if (fq->xdp)
+		return false;
+
+	/* It should be either a header pool or a ZC payload pool */
+	if (fq->type == LIBETH_FQE_HDR)
+		return !fq->hsplit;
+
+	pp->flags |= PP_FLAG_ALLOW_UNREADABLE_NETMEM;
+
+	return fq->hsplit;
+}
+
 /**
  * libeth_rx_fq_create - create a PP with the default libeth settings
  * @fq: buffer queue struct to fill
@@ -166,6 +208,9 @@ int libeth_rx_fq_create(struct libeth_fq *fq, void *napi_dev)
 	struct page_pool *pool;
 	int ret;
 
+	if (!libeth_rx_page_pool_check_unread(fq, &pp))
+		return -EINVAL;
+
 	pp.dma_dir = fq->xdp ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
 
 	if (!fq->hsplit)
-- 
2.52.0