Re: [PATCH] input/spi: add ads7843 support to ads7846 touchscreen driver

[Nicolas Ferre <nicolas.ferre@rfo.atmel.com>]

Nicolas Ferre a écrit :
> > > As the SPI underlying code behaves quite differently from a 
> > > controller driver
> > > to another whan not having a tx_buf filled, I have add a zerroed 
> > > buffer to give
> > > to the spi layer while receiving data.
> >
> > You must be working with a buggy controller driver then.  That part of
> > this patch should never be needed.  It's expected that rx-only transfers
> > will omit a tx buf; all controller drivers must handle that case.
>
> I said that because it is true that most of spi controller drivers 
> manage rx only transactions filling the tx buffer with zerros but the 
> spi_s3c24xx.c driver seems to fill with ones (line 177 hw_txbyte())
>
> Anyway, I will check in our controller driver to sort this out.

I dug a bit into this.
Well, in the atmel_spi driver code, we use previous rx buffer if we do 
not provide a tx_buf (as it is said that in struct spi_transfer 
comments,  "If the transmit buffer is null, undefined data will be 
shifted out while filling rx_buf").
So, the touchscreen controller sees sometimes a "start" condition (bit 7 
of a control byte). It then takes the control byte and sets trash bits 
as a configuration. I ran into those troubles and add a zerroed buffer 
as tx.

Do you think that shifting zerros out when a tx_buf is not specified is 
the desired behavior ?

Regards,
--
Nicolas Ferre


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