[U-Boot] [PATCH v4] net: fec_mxc: Poll FEC_TBD_READY after polling TDAR

[Marek Vasut <marex@denx.de>]

On Friday, August 22, 2014 at 05:02:37 AM, Li Ye-B37916 wrote:
> Hi Marek,
> 
> On 8/22/2014 1:18 AM, Marek Vasut wrote:
> > On Thursday, August 21, 2014 at 07:10:32 PM, Fabio Estevam wrote:
> >> When testing the FEC driver on a mx6solox we noticed that the TDAR bit
> >> gets always cleared prior then the READY bit is set in the last BD,
> >> which causes FEC transmission to fail.
> >> 
> >> As explained by Ye Li:
> >> 
> >> "The TDAR bit is set when the descriptors are all out from TX ring, but
> >> the descriptor properly is in transmitting not READY.
> > 
> > Again, I do not understand this sentence :-(
> 
> When transmitting data, FEC internal DMA reads the TX descriptor and move
> the data from the buffer pointed by TX descriptor to FEC internal FIFO.
> All TX descriptors are managed in a ring. We found the TDAR is cleared at
> DMA starting last descriptor of the ring, not at DMA having last
> descriptor finished. So this bit clears earlier than the READY bit of last
> descriptor. The delay is the time for the data sending of last descriptor.
> 
> >> These are two signals,
> >> and in Ic simulation, we found the TDAR always clear prior than the
> >> READY bit of last BD.
> > 
> > And this is the behavior of which version of the FEC IP, the "old" one or
> > the one present in MX6slx ?
> 
> This is the behavior of current FEC IP on mx6sx. For old ones, we did not
> do simulation for them, but it seems the TDAR clear at the last TX
> descriptor finished.

Thanks for clarification. Fabio, can you please document this with a big comment 
in the code ?

Best regards,
Marek Vasut