Re: [PATCH v2 2/2] spi: fsl-spi: Implement trailing bits

[Christophe Leroy <christophe.leroy@xxxxxxxxxx>]

Le 22/08/2022 à 19:15, Mark Brown a écrit :
> On Thu, Aug 18, 2022 at 06:35:39PM +0000, Christophe Leroy wrote:
> 
>> Yes indeed. Therefore in v3 I took a different approach : a flag .cs_off
>> tells to spi_transfer_one_message() that a given transfer has to be
>> performed with chipselect OFF, therefore the consumer has full control
>> of how and when to add those additional fake clock cycles during a
>> transfer, and can eventually add one at anyplace during the transfer.
> 
>> Here an exemple of what will do the consumer.
> 
> Hrm, we should already be able to synthesize that with cs_change though
> there's usability challenges there and AFAICT it doesn't work for the
> first transfer which your proposal would so there's a functional benefit
> even if you don't need it for your device right now.  It would be good
> if you could have a look at using cs_change for your use case.  Sorry, I
> don't think I'd fully realised what you were looking to accomplish here
> until I saw your proposal.

I think we already addressed this possibility back in 2016, see 
https://lore.kernel.org/linux-spi/20160824111206.GD22076@xxxxxxxxxxxxx/

The conclusion was that it was not possible to accomplish it with cs_change.

Or did we miss something at that time ?

My understanding is that if you set cs_change for any transfer of a 
message except the last, then CS goes OFF then ON again between the said 
transfer and the following one. If you set cs_change for the last 
transfer of a message, then CS stays ON until next message instead of 
going OFF as usual.

My need is to transfer a fake byte with CS off at the end of a message. 
I still can't see how cs_change can achieve that.

Thanks
Christophe