linux-kernel - Re: Sluggish AT91 I2C driver causes SMBus timeouts

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Message-ID: <92675fc4-060b-0056-4fe8-3fe3368cb617@axentia.se>
Date:   Fri, 13 Oct 2017 17:01:04 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Ludovic Desroches <ludovic.desroches@...rochip.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: Re: Sluggish AT91 I2C driver causes SMBus timeouts

On 2017-10-13 15:29, Alan Cox wrote:
> On Thu, 12 Oct 2017 13:35:17 +0200
> Peter Rosin <peda@...ntia.se> wrote:
> 
>> Hi!
>>
>> I have encountered an "interesting" bug. It silently corrupts data
>> and is generally nasty...
>>
>> On an I2C bus, driven by the at91 driver and DMA (an Atmel
>> sama5d31 chip), I have an 256 byte eeprom (NXP SE97BTP). I'm using
>> Linux v4.13.
> 
> If your force the transfer to PIO does it behave ? Does the controller in
> fact need to siwtch to PIO for SMBUS ?

Like, what if I disable DMA?

I saw no way to do that, short of short-cutting a few things in the
driver code. So, did that and I cannot tickle the bug. But I don't
know if that makes me safe?

Ludovic, any reason to believe disabling DMA will prevent these
stalls, or will they just appear under different circumstances?

I used this dirty "patch" to i2c-at91.c:at91_twi_configure_dma() for
testing:

-	dev->use_dma = true;
+	//dev->use_dma = true;

Cheers,
Peter