netdev - can: tcan4x5x: spi bits_per_word issue on Raspberry PI

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Message-ID: <f1e2a3fba8604996b16e11f9405542d1@escrypt.com>
Date:   Wed, 14 Aug 2019 09:43:21 +0000
From:   "FIXED-TERM Buecheler Konstantin (ETAS-SEC/ECT-Mu)" 
        <fixed-term.Konstantin.Buecheler@...rypt.com>
To:     "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Dan Murphy <dmurphy@...com>
Subject: can: tcan4x5x: spi bits_per_word issue on Raspberry PI

Hi all,

I am trying to use a tcan4550 together with a Raspberry PI 3 B. I am using the tcan4x5x driver from net-next. 
I always get the following error during startup.
	tcan4x5x spi0.0: Probe failed, err=-22
	tcan4x5x: probe of spi0.0 failed with error -22

I realized that this happens because the Raspberry PI does only support 8/9 bit words. https://elinux.org/index.php?title=RPi_SPI#Supported_bits_per_word
In the driver it is set to 32.
	spi->bits_per_word = 32;

Setting this to 8 does not help of course since the tcan chip expects a multiple of 32 per spi transaction. 
I don't know if this is a Raspberry Pi specific problem or if there are more devices with this hardware limitation. 

Does anyone have a workaround for that? 

If this a common issue it might be a good idea to patch the driver. I will check if I can find proper a way to do so. 

Regards, 
Konstantin