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)

quoted

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 ?

I recheck on my side, it seems like the fix needed for simics model at my
end and not needed for actual SoC. This is strange but I can check internal
team. I also read through the databook which says that for 40-bit or 48-bit
addressing mode, the tail pointer must be advanced to the location
immediately after the descriptors are set, for the DMA to know that
additional descriptors are available.

Now, relooking at the current logic which sets the rx tail pointer according
to the value of "dirty_rx" which can be "zero" as it takes value from entry
that is incremented through STMMAC_GET_ENTRY(entry, DMA_RX_SIZE).
This too can give a situation that the base and tail registers is pointing to
the same location.

According to SNPS databook, the DMA engine goes into SUSPEND state if the
Rx descriptors are not OWN=1. The operation can be resumed by ensuring that
the descriptors are owned by the DMA and then update the tail pointer.

What is your opinion here if we always update the Rx tail pointer to pointer
the boundary of the DMA size as follow without depending on dirty_rx.

rx_q->rx_tail_addr = rx_q->dma_rx_phy + (DMA_RX_SIZE *
		     sizeof(struct dma_desc))

Thanks
Boon Leong