stm32h7-eth: Properly maintain Tx tail pointer

Analagous to #1493, we should maintain the Tx tail pointer
properly, as well.  Note that since the device won't examine a
Tx descriptor until the tail pointer is set appropriately, we
can relax stores and rely on the barrier in `tx_notify` to flush
everything to RAM properly before writing the tail pointer and
kicking off a DMA.

drv/stm32h7-eth/src/lib.rs

@@ -149,14 +149,16 @@ impl Ethernet {
         dma.dmacrx_rlr
             .write(|w| unsafe { w.rdrl().bits(rx_ring_len) });
 
-        // Poke both tail pointers so the hardware looks at the descriptors. We
-        // completely initialize the descriptor array, so the tail pointer is
-        // always the end.
+        // Poke the receive tail pointer so that the hardware looks at
+        // descriptors. We completely initialize the descriptor array, so the
+        // tail pointer is always as close to the end as we can make it.
         //
         // Doing the same drop-bottom-two-bits stuff that we had to do for DLARs
         // above.
-        dma.dmactx_dtpr
-            .write(|w| unsafe { w.tdt().bits(tx_ring.tail_ptr() as u32 >> 2) });
+        //
+        // We don't set the transmit tail pointer until we enqueue a packet,
+        // as we don't want the hardware to race against software filling in
+        // descriptors.
         dma.dmacrx_dtpr
             .write(|w| unsafe { w.rdt().bits(rx_ring.tail_ptr() as u32 >> 2) });
 
@@ -334,7 +336,7 @@ impl Ethernet {
         // Poke the tail pointer so the hardware knows to recheck (dropping two
         // bottom bits because svd2rust)
         self.dma.dmactx_dtpr.write(|w| unsafe {
-            w.tdt().bits(self.tx_ring.tail_ptr() as u32 >> 2)
+            w.tdt().bits(self.tx_ring.next_tail_ptr() as u32 >> 2)
         });
     }
 

drv/stm32h7-eth/src/ring.rs

@@ -144,12 +144,12 @@ impl TxRing {
         // ensure that they're owned by us (not the hardware).
         for desc in storage {
             #[cfg(not(feature = "vlan"))]
-            desc.tdes[3].store(0, Ordering::Release);
+            desc.tdes[3].store(0, Ordering::Relaxed);
 
             #[cfg(feature = "vlan")]
             {
-                desc.tdes[0][3].store(0, Ordering::Release);
-                desc.tdes[1][3].store(0, Ordering::Release);
+                desc.tdes[0][3].store(0, Ordering::Relaxed);
+                desc.tdes[1][3].store(0, Ordering::Relaxed);
             }
         }
         Self {
@@ -165,12 +165,10 @@ impl TxRing {
         self.storage.as_ptr()
     }
 
-    /// Returns a pointer to the byte just past the end of the `TxDesc` ring.
-    /// This too gets loaded into the DMA controller, so that it knows what
-    /// section of the ring is initialized and can be read. (The answer is "all
-    /// of it.")
-    pub fn tail_ptr(&self) -> *const TxDesc {
-        self.storage.as_ptr_range().end
+    /// Returns a pointer to the "next" descriptor in the ring.  We load this
+    /// into the device so that the DMA engine knows what descriptors are free.
+    pub fn next_tail_ptr(&self) -> *const TxDesc {
+        self.base_ptr().wrapping_add(self.next.get())
     }
 }
 
@@ -247,7 +245,7 @@ impl TxRing {
                 | 1 << TDES3_LD_BIT
                 | TDES3_CIC_CHECKSUMS_ENABLED << TDES3_CIC_BIT
                 | len as u32;
-            d.tdes[3].store(tdes3, Ordering::Release); // <-- release
+            d.tdes[3].store(tdes3, Ordering::Relaxed);
 
             self.next.set(if self.next.get() + 1 == self.storage.len() {
                 0
@@ -322,7 +320,7 @@ impl TxRing {
                 | 1 << TDES3_CTXT_BIT
                 | 1 << TDES3_VLTV_BIT
                 | u32::from(vid);
-            d.tdes[0][3].store(tdes3, Ordering::Release); // <-- release
+            d.tdes[0][3].store(tdes3, Ordering::Relaxed); // <-- release
 
             // Program the tx descriptor to represent the packet, using the
             // same strategy as above for memory access ordering.
@@ -335,7 +333,7 @@ impl TxRing {
                 | 1 << TDES3_LD_BIT
                 | TDES3_CIC_CHECKSUMS_ENABLED << TDES3_CIC_BIT
                 | len as u32;
-            d.tdes[1][3].store(tdes3, Ordering::Release); // <-- release
+            d.tdes[1][3].store(tdes3, Ordering::Relaxed);
 
             self.next.set(if self.next.get() + 1 == self.storage.len() {
                 0