linux-kernel - RE: [PATCH v7 0/5] AT91: replace broken TWI driver i2c-at91.c

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Date:	Wed, 11 Jan 2012 15:06:52 +0100
From:	"Voss, Nikolaus" <N.Voss@...nmann.de>
To:	"'Carsten Behling'" <carsten.behling@...z-fricke.com>,
	"'Hubert Feurstein'" <h.feurstein@...il.com>
CC:	"'linux-i2c@...r.kernel.org'" <linux-i2c@...r.kernel.org>,
	"'linux-arm-kernel@...ts.infradead.org'" 
	<linux-arm-kernel@...ts.infradead.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'ben-linux@...ff.org'" <ben-linux@...ff.org>
Subject: RE: [PATCH v7 0/5] AT91: replace broken TWI driver i2c-at91.c

Hi,

Carsten Behling wrote on 2011-12-28:
> I've tested this driver with the pca953x GPIO expander driver with a PCA9554.
> 
> In the case of 8 GPIO pins (my case) "i2c_smbus_read_byte_data(...)"
[...]
> I observed that reading out the GPIO status is one read delayed.
> The first read to a register from the pca953x driver does not result in a
> RXRDY interrupt and at91_twi_read_next_byte(...) is not called.
> Only the completion routine is triggered with a TXCOMP interrupt.

Which SOC are you using? This is probably a hardware bug since on my
at91sam9g45 i2c_smbus_read_byte_data() works reliably without any
problems. Please check the errata and see if there is a useable
workaround. If not, the driver should be disabled for your SOC.

> Additionally, I took a look at the status flags just after the control flags
> are set (in at91_do_twi_transfer(...), AT91_TWI_START|AT91_TWI_STOP for the
> one byte read). Surprisingly, the AT91_TWI_RXRDY flag is zero before the first
> register read and one for the following reads. According to the manual this
> flag should be zero after a read on AT91_TWI_RHR.
> 
> Reading the AT91_TWI_RHR before the control flags are set to be sure that the
> AT91_TWI_RXRDY is cleared leads to a never occurring RXRDY interrupt.

Again, this behavior does not conform to the datasheet, I suspect documented
or undocumented errata... At91rm9200 has at least one erratum for which I
imported a workaround from the old driver (which does not use interrupts).

Niko

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