linux-kernel - Re: [PATCH 1/2] mfd: cros ec: spi: Don't send first message too soon

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Date:   Tue, 14 Nov 2017 14:44:31 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     Brian Norris <briannorris@...omium.org>
CC:     Lee Jones <lee.jones@...aro.org>, <linux-kernel@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>,
        Shawn Nematbakhsh <shawnn@...omium.org>,
        Benson Leung <bleung@...omium.org>,
        Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH 1/2] mfd: cros ec: spi: Don't send first message too soon


On 13/11/17 21:19, Brian Norris wrote:
> + others
> 
> On Mon, Nov 13, 2017 at 09:05:39PM +0000, Jon Hunter wrote:
>> On the Tegra124 Nyan-Big chromebook the very first SPI message sent to
>> the EC is failing.
>>
>> The Tegra SPI driver configures the SPI chip-selects to be active-high
>> by default (and always has for many years). The EC SPI requires an
>> active-low chip-select and so the Tegra chip-select is reconfigured to
>> be active-low when the EC SPI driver calls spi_setup(). The problem is
>> that if the first SPI message to the EC is sent too soon after
>> reconfiguring the SPI chip-select, it fails.
>>
>> The EC SPI driver prevents back-to-back SPI messages being sent too
>> soon by keeping track of the time the last transfer was sent via the
>> variable 'last_transfer_ns'. To prevent the very first transfer being
>> sent too soon, initialise the 'last_transfer_ns' variable after calling
>> spi_setup() and before sending the first SPI message.
>>
>> Signed-off-by: Jon Hunter <jonathanh@...dia.com>
>> ---
>> Looks like this issue has been around for several Linux releases now
>> and it just depends on timing if this issue is seen or not and so there
>> is no specific commit this fixes. However, would be good to include for
>> v4.15.
> 
> I wonder if that doesn't mean we should have a stable tag still?
> 
> Cc: <stable@...r.kernel.org>

I was thinking about that as well after I sent it. I have resent as a V2
CC'ing stable.

Cheers
Jon

-- 
nvpublic