netdev - Re: M_CAN message RAM initialization AppNote - was: Re: [PATCH V3 3/3] can: m_can: workaround for transmit data less than 4 bytes

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Date:	Thu, 06 Nov 2014 10:00:34 +0100
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Oliver Hartkopp <socketcan@...tkopp.net>,
	Dong Aisheng <b29396@...escale.com>
CC:	linux-can@...r.kernel.org, wg@...ndegger.com,
	varkabhadram@...il.com, netdev@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: M_CAN message RAM initialization AppNote  - was: Re: [PATCH V3
 3/3] can: m_can: workaround for transmit data less than 4 bytes

On 11/06/2014 08:04 AM, Oliver Hartkopp wrote:
> On 06.11.2014 02:57, Dong Aisheng wrote:
>> On Wed, Nov 05, 2014 at 07:15:10PM +0100, Oliver Hartkopp wrote:
> 
>>> The Message RAM is usually equipped with a parity or ECC functionality.
>>> But RAM cells suffer a hardware reset and can therefore hold
>>> arbitrary content at startup - including parity and/or ECC bits.
>>>
>>> So when you write only the CAN ID and the first four bytes the last
>>> four bytes remain untouched. Then the M_CAN starts to read in 32bit
>>> words from the start of the Tx Message element. So it is very likely
>>> to trigger the message RAM error when reading the uninitialized
>>> 32bit word from the last four bytes.
>>>
>>> Finally it turns out that an initial writing (with any kind of data)
>>> to the entire message RAM is mandatory to create valid parity/ECC
>>> checksums.
>>>
>>> That's it.
>>>
>>
>> Thanks for sharing this information.
>> Does it mean this issue is related to the nature of Message RAM and is
>> supposed to exist on all M_CAN IP versions?
> 
> From what I know from the 3.1.x revision there's no change regarding
> IR.BRU and IR.BEC - so I would assume this to stay on all M_CAN IP
> revisions.
> 
> But after some sleep I wonder if this patch [3/3] would need an update too.
> 
> Writing to the TX message RAM is obviously no workaround but a valid and
> needed initialization process.
> 
> I would tend to make this patch:
> 
> ---
> 
> can: m_can: add missing TX message RAM initialization
> 
> The M_CAN message RAM is usually equipped with a parity or ECC
> functionality.
> But RAM cells suffer a hardware reset and can therefore hold arbitrary
> content at startup - including parity and/or ECC bits.
> 
> To prevent the M_CAN controller detecting checksum errors when reading
> potentially uninitialized TX message RAM content to transmit CAN frames
> the TX message RAM has to be written with (any kind of) initial data.
> 
> ---
> 
> Then the code should memset() the entire TX FIFO element - and not only
> the 8 data bytes we are addressing now.

No literal memset() as this is iomem

Marc

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