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

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Date:	Thu, 21 Dec 2006 15:40:11 +0100
From:	Nicolas Ferre <nicolas.ferre@....atmel.com>
To:	David Brownell <david-b@...bell.net>
CC:	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Patrice Vilchez <patrice.vilchez@....atmel.com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: 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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