linux-kernel - Re: [PATCH v3 1/5] spi: spi-mem: Add driver for NXP FlexSPI controller

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Message-ID: <20181008143240.00e784a8@bbrezillon>
Date:   Mon, 8 Oct 2018 14:32:40 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Yogesh Narayan Gaur <yogeshnarayan.gaur@....com>
Cc:     "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "marek.vasut@...il.com" <marek.vasut@...il.com>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "robh@...nel.org" <robh@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "computersforpeace@...il.com" <computersforpeace@...il.com>,
        "frieder.schrempf@...eet.de" <frieder.schrempf@...eet.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/5] spi: spi-mem: Add driver for NXP FlexSPI
 controller

On Mon, 8 Oct 2018 11:21:13 +0000
Yogesh Narayan Gaur <yogeshnarayan.gaur@....com> wrote:

> > > +static void nxp_fspi_read_ahb(struct nxp_fspi *f, const struct
> > > +spi_mem_op *op) {
> > > +	u32 len = op->data.nbytes;
> > > +
> > > +	/* Read out the data directly from the AHB buffer. */
> > > +	memcpy_fromio(op->data.buf.in, (f->ahb_addr + op->addr.val), len);  
> > 
> > Don't know if it's supported, but if it is, I recommend using DMA to do this copy,
> > because otherwise you might stall the CPU for quite a long time if the flash is
> > operating in a low-speed mode, and RT maintainers will complain about that at
> > some point ;-).
> >   
> Read using DMA is not supported by the controller in AHB mode, only supported in IP mode.
> Have to use memcpy_fromio() calls. Maximum data size can be read in single call is 0x800 using AHB read.
> 

Still, blocking the CPU until the SPI controller has read 0x800 bytes
is enough to make you miss a real-time deadline. Don't you have a
controller that supports mem2mem transfers (I'm pretty sure a mem2mem
DMA transfer would do the trick here)?

Regards,

Boris