Re: Applied "spi: pxa2xx: Deal with the leftover garbage in TXFIFO" to the spi tree

[Lubomir Rintel <lkundrak@xxxxx>]

On Tue, 2019-03-19 at 11:34 +0200, Andy Shevchenko wrote:
> On Tue, Nov 13, 2018 at 8:37 PM Mark Brown <broonie@xxxxxxxxxx> wrote:
> 
> > There doesn't seem to be a way to empty TXFIFO on MMP2. The datasheet is
> > super-secret and the method described in Armada 16x manual won't work:
> > 
> >   "The TXFIFO and RXFIFO are cleared to 0b0 when the SSPx port is reset or
> >   disabled (by writing a 0b0 to the <Synchronous Serial Port Enable> field
> >   in the SSP Control Register 0)."
> > 
> >   # devmem 0xd4037008           # read SSSR
> >   0x0000F204
> >   # devmem 0xd4037000 32 0x07   # SSE off in SSCR0
> >   # devmem 0xd4037000 32 0x87   # SSE on
> >   # devmem 0xd4037008
> >   0x0000F204
> >          ^ TXFIFO level is still 2. Sigh.
> > 
> > The OLPC 1.75 boot firmware leaves two bytes in the TXFIFO. Those are
> > basically throwaway bytes used in response to the messages from the EC.
> > The OLPC kernel copes with this by power-cycling the hardware. Perhaps
> > the firmware should do this instead.
> > 
> > Other than that, there's not much we can do other than complain loudly
> > until the garbage gets drained and discard the actual data... For the
> > OLPC EC this will work just fine and pushing more data to TXFIFO would
> > break further transactions.
> > @@ -1078,6 +1078,20 @@ static int pxa2xx_spi_transfer_one(struct spi_controller *master,
> 
> Hmm...  Shouldn't be enough to do this in ->setup() or even in
> ->probe() / ->resume()?

I can't see how, though. As far as I can tell there's no way to just
dump bytes from the TXFIFO. In slave mode they just stay there until
the master initiates a transfer. The only solution I can think of is to
avoid pushing more data there until we're in sync.

Lubo