linux-kernel - RE: [PATCH 1/2] spi: spi-mem: Add the spi_set_xfer

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Message-ID: <AM6PR04MB435700CF27EE20D9271C0A2397EC0@AM6PR04MB4357.eurprd04.prod.outlook.com>
Date:   Fri, 28 Sep 2018 07:29:18 +0000
From:   Chuanhua Han <chuanhua.han@....com>
To:     Boris Brezillon <boris.brezillon@...tlin.com>
CC:     "broonie@...nel.org" <broonie@...nel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "eha@...f.com" <eha@...f.com>
Subject: RE: [PATCH 1/2] spi: spi-mem: Add the spi_set_xfer_bpw function



> -----Original Message-----
> From: Boris Brezillon <boris.brezillon@...tlin.com>
> Sent: 2018年9月28日 15:19
> To: Chuanhua Han <chuanhua.han@....com>
> Cc: broonie@...nel.org; linux-spi@...r.kernel.org;
> linux-kernel@...r.kernel.org; eha@...f.com
> Subject: Re: [PATCH 1/2] spi: spi-mem: Add the spi_set_xfer_bpw function
> 
> On Fri, 28 Sep 2018 06:59:58 +0000
> Chuanhua Han <chuanhua.han@....com> wrote:
> 
> > >
> > > It's still unclear why you need to specify a bits_per_word value,
> > > but if this is needed, it's probably something you want to add to
> > > spi.c, when a message is queued.
> > To specify a specific bits_per_word to be able to use the xspi
> > (32bit) mode of the fsl_dspi module to transfer data, you can look at
> > my PATCH 2/2. Do not add a value in spis.c that takes into account
> > that the value assigned to bits_per_word is decided before the
> > transfer. Thanks for your check and reply!
> 
> I might be wrong, but that's not my understanding of ->bits_per_word.
> To me, it's something that you can use when your *device* (not
> controller) expects non-byte aligned words [1]. The spi-mem protocol is
> definitely designed to work with 1byte large words, so, as I said, I suspect
> you're abusing xfer->bits_per_word to address a controller driver issue.
> 
> [1]https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Feli
> xir.bootlin.com%2Flinux%2Flatest%2Fsource%2Finclude%2Flinux%2Fspi%2Fspi
> .h%23L114&amp;data=02%7C01%7Cchuanhua.han%40nxp.com%7C7ec61d6d
> 7ef741aba84408d62512a9e1%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0
> %7C0%7C636737159420820050&amp;sdata=iwFgXG3yFvH7Ruac7MGCoaP8h
> l2M916m9%2BZeV7nksTg%3D&amp;reserved=0
Ah, I'll study it. Thank you！