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

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Date:   Fri, 28 Sep 2018 09:18:33 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Chuanhua Han <chuanhua.han@....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

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://elixir.bootlin.com/linux/latest/source/include/linux/spi/spi.h#L114