RE: [PATCH net 1/7] net: stmmac: fix error in updating rx tail pointer to last free entry

From: Ong Boon Leong <redacted>
Date: Jan/14/2020, 02:01:10 (UTC+00:00)

DMA_CH(#i)_RxDesc_Tail_Pointer points to an offset from the base and
indicates the location of the last valid descriptor.

The change introduced by "net: stmmac: Update RX Tail Pointer to last
free entry" incorrectly updates the RxDesc_Tail_Pointer and causess
Rx operation to freeze in corner case. The issue is explained as
follow:-

Say, cur_rx=1 and dirty_rx=0, then we have dirty=1 and entry=0 before
the while (dirty-- > 0) loop of stmmac_rx_refill() is entered. When
the while loop is 1st entered, Rx buffer[entry=0] is refilled and after
entry++, then, entry=1. Now, the while loop condition check "dirty-- > 0"
and the while loop bails out because dirty=0. Up to this point, the
driver code works correctly.

However, the current implementation sets the Rx Tail Pointer to the
location pointed by dirty_rx, just updated to the value of entry(=1).
This is incorrect because the last Rx buffer that is refileld with empty
buffer is with entry=0. In another words, the current logics always sets
Rx Tail Pointer to the next Rx buffer to be refilled (too early).

So, we fix this by tracking the index of the most recently refilled Rx
buffer by using "last_refill" and use "last_refill" to update the Rx Tail
Pointer instead of using "entry" which points to the next dirty_rx to be
refilled in future.

I'm not sure about this ...

RX Tail points to last valid descriptor but it doesn't point to the base 
address of that one, it points to the end address.

Let's say we have a ring buffer with just 1 descriptor. With your new 
logic then: RX base == RX tail (== RX base), so the IP will not see any 
descriptor. But with old logic: RX base == (RX base + 1), which causes 
the IP to correctly see the descriptor.

Can you provide more information on the Rx operation freeze you 
mentioned ? Can it be another issue ?

---
Thanks,
Jose Miguel Abreu

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