linux-kernel - Re: [PATCH 16/17] spi: dw: Fix Rx-only DMA transfers

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Date:   Fri, 8 May 2020 22:27:10 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Mark Brown <broonie@...nel.org>,
        Serge Semin <fancer.lancer@...il.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>,
        Allison Randal <allison@...utok.net>,
        Gareth Williams <gareth.williams.jx@...esas.com>,
        Rob Herring <robh+dt@...nel.org>, linux-mips@...r.kernel.org,
        devicetree@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Xinwei Kong <kong.kongxinwei@...ilicon.com>,
        Thor Thayer <thor.thayer@...ux.intel.com>,
        Charles Keepax <ckeepax@...nsource.cirrus.com>,
        "wuxu.wu" <wuxu.wu@...wei.com>,
        Phil Edworthy <phil.edworthy@...esas.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/17] spi: dw: Fix Rx-only DMA transfers

On Fri, May 08, 2020 at 04:29:41PM +0300, Serge Semin wrote:
> Tx-only DMA transfers are working perfectly fine since in this case
> the code just ignores the Rx FIFO overflow interrupts. But it turns
> out the SPI Rx-only transfers are broken since nothing pushing any
> data to the shift registers, so the Rx FIFO is left empty and the
> SPI core subsystems just returns a timeout error. Since DW DMAC
> driver doesn't support something like cyclic write operations of
> a single byte to a device register, the only way to support the
> Rx-only SPI transfers is to fake it by using a dummy Tx-buffer.
> This is what we intend to fix in this commit by setting the
> SPI_CONTROLLER_MUST_TX flag for DMA-capable platform.

Hmm... If Mark consider this a right thing to do, then it's fine.
I didn't investigate what this flag may produce as a side effect.

> --- a/drivers/spi/spi-dw.c
> +++ b/drivers/spi/spi-dw.c
> @@ -524,6 +524,7 @@ int dw_spi_add_host(struct device *dev, struct dw_spi *dws)
>  			dws->dma_inited = 0;
>  		} else {
>  			master->can_dma = dws->dma_ops->can_dma;
> +			master->flags |= SPI_CONTROLLER_MUST_TX;
>  		}
>  	}
>  
> -- 
> 2.25.1
> 

-- 
With Best Regards,
Andy Shevchenko