Re: Waiting for the PHY to complete auto-negotiation

+ Mans

On 09/12/2017 19:49, Florian Fainelli wrote:

Having any HW state machine requiring X number of clock cycles to
guarantee a full transition to a stopped state is not unusual, however,
the fact that you need to send 5 packets to guarantee an EOC descriptor
is hit is completely unusual. Ideally there is a single bit that tells
the DMA engine: you are enabled, do your thing, or you are now disabled,
and you must stop all accesses to DRAM *now*.

So what would be the correct way to quiesce that controller according to
your HW folks?

He (it's a single person) offered to run some RTL-level simulations,
but then moved on to more important tasks. At some point he wrote:

If you reset the DMA enable in the middle of a DMA, the hardware
state machine doesn't return to the IDLE state if it has more
descriptors to process. It should be noted that the RX DMA has been
designed by our IP vendor with no intention of stopping the DMA in
the middle of its operation.

While the documentation for the IP states:

Receive DMA Channel Disabling

When the entire receive frame has been read from the Receive FIFO and
sent over the AMBA bus, the DMA operation ends, and the Receive DMA
Channel is automatically disabled.  To do this, hardware resets the
Enable bit in the Receive Channel Control Register to "0" after the
last data has been read from the Receive FIFO and sent over the AMBA
bus.

When operating in descriptor mode, upon completion of a receive frame
DMA operation, if the descriptor chain has not ended when a receive
frame DMA operation completes, the next receive frame DMA operation
begins.  The last descriptor in a descriptor chain is indicated by
having its End Of Chain- EOC, flag set to "1".  If this EOC flag is
"0", to begin the next receive frame DMA operation, the next
descriptor is automatically retrieved and used to configure the
Receive DMA Channel.  The Receive DMA Channel is then automatically
re-enabled and the next receive frame DMA operation begins.

In descriptor mode, an AMBA bus error can occur when reading receive
descriptor data.  If this happens, receive descriptor processing ends
and the Receive DMA Channel is turned off.  The Descriptor Error bit
in the Receive Status Register is set to "1".

I don't see how sending "fake" packets through the loop back would be
considered "resetting the DMA enable in the middle of a DMA".
(I'm afraid the HW dev didn't grasp what the driver is doing.)

I suppose I should test forcefully setting the enable bit to 0 in
the driver, and see if hell breaks loose.

Regards.