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

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Message-ID: <f78bb414-4165-3f56-151a-47ab4a8a645d@pengutronix.de>
Date:   Thu, 15 Aug 2019 10:34:41 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     "FIXED-TERM Buecheler Konstantin (ETAS-SEC/ECT-Mu)" 
        <fixed-term.Konstantin.Buecheler@...rypt.com>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:     Dan Murphy <dmurphy@...com>
Subject: Re: can: tcan4x5x: spi bits_per_word issue on Raspberry PI

On 8/14/19 5:01 PM, FIXED-TERM Buecheler Konstantin (ETAS-SEC/ECT-Mu) wrote:
>> 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?
> 
> It seems to be enough to just change the bits_per_word value to 8
> 
>>
>> 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
> 
> Now I have another really confusing problem. Anything I write to SPI is written little endian. The tcan chip expects big endian. 
> Anything I read from SPI is treated as little endian but is big endian. Does anyone know why this happens? 
> Is there a flag or something I can set for the SPI device/wire to fix this?

Have you changed the bits_per_word to 8? Then you read just a stream of
bytes. If you tread them as an u32 they are in host order.

Marc

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