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

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Message-ID: <20200522121820.GG5801@sirena.org.uk>
Date:   Fri, 22 May 2020 13:18:20 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Serge Semin <fancer.lancer@...il.com>,
        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 03:12:21PM +0300, Andy Shevchenko wrote:
> On Fri, May 22, 2020 at 02:52:35PM +0300, Serge Semin wrote:

> > 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!

Realistically it seems unlikely that the clock will be even as slow as
double digit kHz though, and if we do I'd not be surprised to see other
problems kicking in.  It's definitely good to handle such things if we
can but so long as everything is OK for realistic use cases I'm not sure
it should be a blocker.

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