linux-kernel - Re: [PATCH v4 01/16] spi: dw: Add Tx/Rx finish wait methods to the MID DMA

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Date:   Fri, 22 May 2020 15:12:21 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Serge Semin <fancer.lancer@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Linus Walleij <linus.walleij@...ricsson.com>,
        Vinod Koul <vkoul@...nel.org>, Feng Tang <feng.tang@...el.com>,
        Grant Likely <grant.likely@...retlab.ca>,
        Alan Cox <alan@...ux.intel.com>,
        Georgy Vlasov <Georgy.Vlasov@...kalelectronics.ru>,
        Ramil Zaripov <Ramil.Zaripov@...kalelectronics.ru>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, linux-mips@...r.kernel.org,
        devicetree@...r.kernel.org,
        Wan Ahmad Zainie <wan.ahmad.zainie.wan.mohamad@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        "wuxu.wu" <wuxu.wu@...wei.com>, Clement Leger <cleger@...ray.eu>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 01/16] spi: dw: Add Tx/Rx finish wait methods to the
 MID DMA

On Fri, May 22, 2020 at 02:52:35PM +0300, Serge Semin wrote:
> On Fri, May 22, 2020 at 02:13:40PM +0300, Andy Shevchenko wrote:
> > On Fri, May 22, 2020 at 03:07:50AM +0300, Serge Semin wrote:
> > > Since DMA transfers are performed asynchronously with actual SPI
> > > transaction, then even if DMA transfers are finished it doesn't mean
> > > all data is actually pushed to the SPI bus. Some data might still be
> > > in the controller FIFO. This is specifically true for Tx-only
> > > transfers. In this case if the next SPI transfer is recharged while
> > > a tail of the previous one is still in FIFO, we'll loose that tail
> > > data. In order to fix this lets add the wait procedure of the Tx/Rx
> > > SPI transfers completion after the corresponding DMA transactions
> > > are finished.

...

> > > Changelog v4:
> > > - Get back ndelay() method to wait for an SPI transfer completion.
> > >   spi_delay_exec() isn't suitable for the atomic context.
> > 
> > OTOH we may teach spi_delay_exec() to perform atomic sleeps.
> 
> Please, see it's implementation. It does atomic delay when the delay value
> is less than 10us. But selectively gets to the usleep_range() if value is
> greater than that.

Oh, than it means we may do a very long busy loop here which is not good at
all. If we have 10Hz clock, it might take seconds of doing nothing!

...

> > > +	while (dw_spi_dma_tx_busy(dws) && retry--)
> > > +		ndelay(ns);
> > 
> > I might be mistaken, but I think I told that this one misses to keep power
> > management in mind.
> 
> Here we already in nearly atomic context due to the callback executed in the
> tasklet. What power management could be during a tasklet execution? Again we
> can't call sleeping methods in here. What do you suggest in substitution?
> 
> > Have you read Documentation/process/volatile-considered-harmful.rst ?
> 
> That's mentoring tone is redundant. Please, stop it.

I simple gave you pointers to where you may read about power management in busy
loops. Yes, I admit that documentation title and the relation to busy loops is
not obvious.

-- 
With Best Regards,
Andy Shevchenko