linux-kernel - Re: [PATCH 2/5] spi: implement companion-spi driver

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Date:   Fri, 8 Jun 2018 09:05:05 +0200
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     "Jonas Mark (BT-FIR/ENG1)" <Mark.Jonas@...bosch.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Heiko Schocher <hs@...x.de>,
        "ZHU Yi (BT-FIR/ENG1-Zhu)" <Yi.Zhu5@...bosch.com>
Subject: Re: [PATCH 2/5] spi: implement companion-spi driver

On Fri, Jun 08, 2018 at 06:56:23AM +0000, Jonas Mark (BT-FIR/ENG1) wrote:
> Hi Oleksij,
> 
> > > > > +               if (slave_is_not_busy(priv))
> > > > > +                       return 0;
> > > > > +
> > > >
> > > > > +               udelay(READY_POLL_US_GRAN);
> > > >
> > > > Should it be atomic?
> > > > Can it use read_poll_* type of helpers instead?
> > >
> > > Yes, it shall be atomic because we need to reduce the latency at
> > > detecting the de-assertion of the busy signal. We accept that this will
> > > cost CPU time.
> > >
> > > In case the Companion itself is very busy and does not reply quickly we
> > > are have second polling loop below which sleeps longer and is non-atomic.
> > 
> > I can confirm, this make huge impact on protocol performance. And this
> > protocol is not really the speed runner.
> 
> The challenge is that the protocol is synchronous and without back to
> back transfers.
> 
> > you can send dummy message to set CS.
> > +	struct spi_transfer t = {
> > +		.len = 0,
> > +		.cs_change = 1,
> > +	};
> > 
> > +	/* send dummy to trigger CS */
> > +	reinit_completion(&priv->fc_complete);
> > +	spi_sync_locked(spi, &m);
> > 
> > see my ancient unfinished code:
> > https://patchwork.kernel.org/patch/9418511/
> 
> We will check it out.
> 
> > In general, this part of the code, can be provided as separate driver
> > which should be called as "SPI 5wire protocol". 3 wires for data, 1 -
> > chip select, 1 - busy state. Since, the slave cant fit to normal SPI
> > requirements, and can't be ready within predictable time, busy signal is
> > needed. Providing this as separate driver or part of SPI framework
> > should reduce the code for many different drivers.
> > 
> > The actual datagram on top of your spi companion should go to
> > separate driver. There are similar (almost identical designs)
> > 
> > can :+
> > char:+
> > dw:  +
> > gpio:+--->some_multi_end_mux_protocol--->spi_5wire_protocol->spi--->
> > 
> > In my knowledge, only data format on top of spi_5wire_protocol is
> > different. See my notes for similar topics:
> > https://github.com/olerem/icc
> > https://github.com/olerem/spi-5wire
> 
> With 5-wire protocol you are referencing to CLK, MISO, MOSI, CS (all
> standard SPI signals) and an extra BUSY signal. What we implemented is
> a 6-wire protocol. There is an additional REQUEST line where the SPI
> slave requests a transfer. It is like a level triggered interrupt
> request.
> 
> Yes, for making it more generic the code in drivers/spi/companion
> could be split into a generic 6-wire protocol driver and a multiplexing
> protocol on top of it.
> 
> How does a slave signal that it has data to be picked up with the
> 5-wire protocol?

The request and busy signals are multiplexed to one wire. So, it should
be easy to implement both, 5 and 6 wire protocol in one driver.

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