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

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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